Lawn mower low compression. Here’s What To Do If Your Lawn Mower Won’t Start

Learn the reasons why a lawn mower won’t start after winter or during peak season, and how to fix those problems.

Family Handyman


Most of the time when a lawn mower won’t start the cause is a problem with the gas or the lawn mower carburetor.

What to Do if Your Lawn Mower Won’t Start

Whatever kind of lawn mower you’ve got, the last thing you want once winter finally lifts and spring has sprung is a lawn mower that won’t start.

If you’ve taken the proper steps to winterize your lawn mower, you’re far less likely to be dealing with such issues. It’s also why you should tune up your lawn mower at the start of every season. However, it’s not out of the ordinary to find your gas-powered lawn mower not starting from time to time, so it’s important to know why your lawn mower isn’t starting and how to fix it.

Project step-by-step (6)

Check the Gas Tank

Let’s start with the obvious. Before you have a heart attack pulling on the rip cord, you’ll want to check the fuel. Like any gasoline-powered engine, lawn mowers run out from time to time. Maybe you forgot it was running on fumes when you finished mowing last time. It sounds simple, but we’ve all overlooked the gas tank from time to time.

Even if there is gas in the mower, if the fuel’s been in there more than a month, that could be the problem. Gas sitting around too long in the tank can get contaminated with dirt and extra moisture.

So if your gasoline has been in the mower for more than month, drain the gas properly, dispose of it correctly, and fill up the mower with new gas. It may take quite a few pulls to suck the new gas into the lawn mower carburetor, so be prepared to clean and dry the plug a few more times.

Add fuel stabilizer when you fill up the tank to help protect the gasoline in there from dirt and moisture.

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Check the Spark Plug

Start by making sure the lawn mower spark plug cable is connected to the plug itself. It’s quite possible that it got pulled off there over the winter while the mower was being stored in the garage.

If that’s not the issue, the next step is to remove the spark plug to see if it’s wet. There’s no way the engine will start if it is. So clean the plug with carburetor cleaner and let it dry. Cleaning it with compressed air isn’t enough; you need a solvent to remove oil residue. If it’s really grimy and dirty, it might be best to change the spark plug.

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Check for Debris in the Mower Deck

Grass clippings can get clogged in the mower deck, which can prevent the blade from turning. This is a common problem if you’ve cut wet grass or let the lawn get especially long and bushy between cuttings. If the cord is hard to pull, that’s a good sign that there’s debris clogging up your mower’s deck.

This is a pretty easy problem to solve. With the mower off, flip it on its side or upside down and scrape out the gummed up grass clippings. Once that’s done, you can flip it back over and start it up again.

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Check the Air Filter

The lawn mower’s carburetor regulates the mix of gasoline and air going into the engine where it’s burned to create power. Before air goes into the carburetor it passes through the air filter which prevents dirt and debris from getting into it.

If the air filer is clogged or dirty, it throws the ratio out of whack. Sometimes that results in your lawn mower smoking, and sometimes it prevents it from starting entirely. So take a look at the air filter to see if it’s dirty. If so, you can clean it or just change it outright.

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Check the Carburetor

Another common reason for a lawn mower that won’t start is a clogged or dirty carburetor. It can also cause your mower to run rough or spew black smoke when you’re trying to cut the grass. If that’s the case, you may need to clean the carburetor.

To get to the carburetor, you’ll have to remove the air filter. Once that’s out of the way, you can remove the carburetor in order to clean it.

Once it’s out, check for corrosion. If you see chalky/powdery white corrosion like this, it’s probably better just to replace it. To clean it, take it apart and spray carburetor cleaner on the parts and inside the housing. After that, put the carburetor back together and reinstall it in the mower.

Check the Fuel Filter

Like the air filter, the fuel filter prevents dirt and debris from getting into the combustion chamber of your lawn mower’s engine, taking that stuff out before the gas gets mixed with air in the carburetor. Problems with the fuel filter might also result in the engine sputtering or rough idling, even before it gets to the point of preventing the mower from starting.

To start, tap the side of the carburetor to help the flow of gas. If that doesn’t work, you might have a clogged filter.

Not all lawn mowers have a fuel filter, but for the ones that do, it’s usually located in the fuel line or the fuel tank. To find out where the fuel filter is at, check your lawn mower’s owners manual, which will also tell you what type of filter it is.

If the filter is in the fuel tank, you’ll need to drain the gas from the mower into a drain pan, assuming you can’t run the engine until it’s out of gas. If the filter is in the fuel line, clamp off the fuel line before removing the filter. Once you have the filter off, you can check to see if it’s dirty and clogged by holding it up the light. If it is, install a new one. Make sure it works with this lawn mower maintenance checklist.

Lawn mower won’t start troubleshooting video: fuel, ignition and compression problems video

A clogged air filter, bad spark plug or contaminated gas can prevent your walk-behind lawn mower from starting. Troubleshoot the problem with this Sears PartsDirect video, which shows how to check engine compression, clean the fuel line and look at the choke linkage, return spring and carburetor.

Check out our DIY Walk Behind Mower Repair page for more troubleshooting videos, repair guides and answers to commonly asked questions.

Today we’re talking about some troubleshooting steps to figure out why your lawn mower won’t start. Although your mower might look different from this Craftsman 21-inch mower, they all work pretty much the same.

These are the tools and supplies you might need, depending on the problem.

Tools and equipment needed

  • Starting fluid
  • Fresh gasoline
  • Ratchet and deep socket
  • Spark plug tester
  • Compression gauge
  • Shop rag
  • Fuel-safe container
  • Pipe cleaner
  • Carburetor or carburetor rebuild kit

Fuel, spark and compression

Since we’ll be working around gasoline, choose a well-ventilated area free of open flame or sparks.

For an engine to start and run when you pull the recoil starter rope, it needs 3 things. The first is fuel, which is a combination of gasoline and air that mixes in the carburetor. Then it needs a spark from the spark plug to ignite that fuel in the cylinder. Finally, it needs the right amount of compression in the cylinder to drive the piston. To fix your problem, we’ll have to figure out which of these three is missing.

We’re going to start by checking a few easy things.

Check the air filter

A clogged air filter can keep air from getting to the carburetor and mixing with the gas.

Remove the air filter cover and pull out the air filter. This mower has a pleated paper filter, but yours might be a different type of paper filter, or it might be foam. If you have a paper filter that’s just a little dirty, you can tap the filter on a hard surface to clean it. If it’s completely clogged, then you need to replace it. If your mower has a foam air filter, you can find cleaning instructions in your owner’s manual.

Check for a fuel problem

While the air filter is removed, let’s do a quick test to see if the engine has a fuel problem. Spray starting fluid through the air filter opening to the carburetor and then try to start the engine.

If it doesn’t start, that eliminates a fuel problem, leaving spark and compression on our list of suspects. If the engine starts and runs briefly, click here to skip to the section about diagnosing fuel problems.

Spark plug and compression

Check the spark plug first. Pull off the spark plug wire and remove the spark plug, using a ratchet fitted with a deep socket. Look for carbon or oil buildup on the spark plug electrode that could prevent sparking. Also, look for a crack in the ceramic insulator. If you see excessive buildup or a crack, replace the spark plug.

If the spark plug looks good, reinstall it and then connect a spark plug tester to check the ignition system. Release the rope from the mower handle so it’s in reach when testing the spark plug. Connect the tester boot to the spark plug and connect the spark plug wire to the other end of the tester. Clamp the bail control bar down to release the blade brake. Pull the starter rope and see if the tester sparks. If the tester shows that there’s no spark, follow the steps in our video about troubleshooting the ignition system.

If the spark plug is good and the ignition system works, check the cylinder compression. You need a compression gauge for this test.

First, remove the spark plug from the cylinder and pull the starter rope several times to purge fuel or oil from the cylinder. Insert a compression gauge in the spark plug hole and push the button to zero-out the gauge. Pull the starter rope repeatedly, until the needle on the gauge stops rising. The gauge should measure between 40 and 60 psi of compression. If it’s lower than 40 psi, have a service technician examine the engine and fix the compression problem. The technician has the right tools and the expertise to diagnose low compression.

Testing the gas, tank

Now, if the engine started and ran briefly when you did the starting fluid test, you know the spark and compression are okay and the problem is in the fuel system. For these tests, be sure you’re in a well-ventilated area where gas fumes can’t build up.

First, let’s look at the condition of the gas. Old gas that has absorbed water or gas that has dirt in it can keep a mower from starting. If your gas has been sitting around a while—either in the tank or in the can—drain the tank and fill it with fresh, clean gas from a clean gas can.

In the future, if you’re going to leave gas in the tank for a while, add fuel stabilizer.

To drain the tank, lay a shop rag on the mower deck below the tank to catch spills. Place a clean, durable container under the fuel line connection on the bottom of the tank. Release the fuel line clamp and pull the fuel line off to drain the gas into the container. For information on disposing the gas safely and legally, check with your local fire department or hazardous waste recycling center.

If you find debris in the drained gas, check for a contaminated fuel tank. Use a flashlight to look inside the tank for grass or debris. If you see debris, release the rope from the mower handle and then remove the three screws from the lower housing. Remove the blower housing and pull the tank off the engine. Rinse out the tank with water and let the tank dry completely before reassembling the mower. Don’t pour that water down a drain. Dispose of it correctly along with the old gas.

Checking the filter screen, fuel Line

After rinsing out the fuel tank, check the filter screen at the bottom of the gas tank for damage. On this style of engine, the screen isn’t available as a separate part, so if the screen is damaged, you have to replace the tank. Some tanks have a removable fuel filter that you can clean or replace. Check your owner’s manual for more information.

You should also check the fuel line for a crack or clog. Disconnect the other end of the fuel line from the carburetor. Examine it for cracks or cuts and replace it if it’s damaged. If you don’t see any damage, look through the fuel line and clear any clogs with a pipe cleaner.

Determining carburetor issues

If you’ve ruled out the quality of the gas and the fuel line, it’s time to turn to the carburetor, which mixes gas and air that is drawn into the cylinder.

Let’s start with the carburetor choke plate. The choke plate controls the ratio of gas and air sent to the carburetor depending on engine temperature. The engine on a lawn mower that’s been idle for more than an hour is cold and needs less air to start. In this case, the choke plate should close to temporarily restrict airflow through the carburetor so more fuel can enter the cylinder.

A choke plate that’s stuck open could prevent the cold engine from starting. Check the choke plate by looking through the carburetor air inlet on the air filter base. The choke plate should be fully closed. If the choke plate is open, check the choke thermostat and linkage. First, remove the air filter base to access the choke linkage and choke thermostat. Remove the blower housing if you didn’t already remove it to rinse out the fuel tank.

Examine the choke linkage and return spring to see if they’re connected properly and moving freely. Reconnect the choke linkage and return spring and replace any broken parts. If the choke linkage and return spring looked good, then your choke thermostat is likely stuck. Replace the choke thermostat if it’s stuck in the open position.

Instead of a choke plate, some engines have a primer bulb that you press to force more fuel into the cylinder when starting a cold engine. Replace the primer bulb if it’s damaged.

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Replace or rebuild carburetor

If the choke plate or primer bulb work, it’s likely you need to replace or rebuild the carburetor. Here‘s a video that shows you how to replace the carburetor.

Or, you could rebuild the carburetor, which takes more effort but costs less. Order the carburetor rebuild kit and then clean and rebuild the carburetor if you want to save money on parts. Here‘s a video that shows how to rebuild the carburetor.

Once you restore the fuel flow to the engine cylinder by replacing or rebuilding the carburetor, the engine should start.

I hope that this video helps you out today. Be sure to check out our other videos, here on the Sears PartsDirect YouTube Channel, and subscribe if you like them.

Lawnmower won’t start when hot, what to do

On a nice day you take your trustworthy lawn mower outside with the intention to trim your lawn. But somehow your Lawnmower is not running smoothly. You discover that it starts and runs fine when cold. However, starting it again when it is hot is an awful experience.

So the question is why is the mower difficult to start when hot? The main culprit is presumably a failing coil, but there can be other reasons as well. In this article, we will look at the various reasons and how to resolve the situation.

Lawnmower won’t start when hot:

  • A Defective Spark Plug not igniting the fuel
  • High/Low oil level
  • Flawed fins foundation of Overheated Engine
  • Loose nuts and bolts become the basis for leakage of air, causing insufficient air supply to the combustion.
  • A restricted Carburetor due to a faulty auto choke causing engine flooding
  • Problematic Compression due to inoperative valve
  • Presence of alcohol in oil.
  • Faulty ignition coil.

If you are unsure how to troubleshoot the problem, we have broken down the steps in simple and user-friendly instructions.

Common troubleshooting steps when your lawnmower does not start when it is hot:

We have identified seven reasons why your Lawn Mower will not, or is difficult to start when it is hot.

Defective Spark Plug

One of the most common causes of lawnmower engine not cranking is due to a faulty spark plug. There is a minimal gap between the two electrodes of the spark plug. It ignites across that gap and aids the proper air.gasoline mixture to combust and produce power. If this gap does not fulfill the optimum length requirement, the combusted heat widens the gap enough to shut down the mower. The spark plug’s spacing should be fixed according to the details given in your Lawnmower’s manufacturing manual. over, the electrode can also weaken due to wear and tear.

Secondly, the buildup soot can also affect spark creation, and this incomplete ignition gives a hard time for the engine to start when hot. The hot start failures are mostly due to the wrong plug fitting. As we already know that the lengths and heat ranges are specified for spark plugs. Initially, you must inspect the correct plug code with your engine maker. It is an excellent practice to have a spare spark plug with you as it permits you to fix your problem swiftly by restoring the plug with a good one. After examining the sparking plug, if you find its components are not smooth, you will see that the issue is resolved, and your time is saved just by restoring the plug.

You can also not repair a defective spark plug; it is recommended to replace your spark plug at the start of every season.

Faulty plug wire

Plug wires are copper coils looped around a metal core. They work as the high voltage passes through them, searching for ground; positive voltage bounces from the plug electrode to the ground. The spark plug provides a perfect path to ground the voltage and spark ignites. This process took a couple of milliseconds. If we provide a relatively shorter path to the ground for the coil voltage, it will eventually shut down the mower engine because the voltage is not reaching the plug.

Therefore, when we emancipate the bail lever, the coil voltage achieves an ideal track to ground, the engine shuts off suddenly. And everyone wants a short cut whether it is human or machine. The engine will not run intermittently if any abrasion is faced by coil control wire against the ground source.

When the engine starts, massive combustion occurs; high-temperature values cause’ high resistance to the voltage flow, which is why plug wires generally breakdown when the mower is hot and begins operating again when the engine cools down. A precautionary measure always keeps your lawn mowers in ventilated areas because the greatest enemy of plug wire and electrical components is moisture.

Malfunctioned Carburetor

Your engine requires fuel or is getting too much air in a failing carburetor, which is the biggest obstacle in its smooth running? We’ll try to figure it out here.

– Does the engine gets enough fuel

Let us check whether the engine is lacking fuel or not. Lawnmowers may be fitted with different choke types (auto choke, manual choke, or a priming bulb) to start a cold engine and smoothly enrich the fuel mixture. Usually, the auto choke carburetors are prone to supplying extra fuel to the cylinder when starting hot; this phenomenon floods the spark plug with fuel that leads to a no start. The gummed-up and sticky choke located inside the Carburetor is another cause for the hindrance in the starting engine, as it fails to provide the proper fuel ratio. Because it malfunctions, the engine stops working.

– Does the engine gets enough air

Another origin is the lack of ample air or provision of extra air for the flooded engine. The auxiliary air could be from a vacuum leak somewhere in the engine or a carburetor fault, whereas the air’s absence might be due to clogged air filters. For the appropriate AFR, remove the air filter and clean the air filters and chokes; otherwise, the air-fuel ratio is off. Gas engines perform their best when the air-fuel ratio is 14.7 to 1 that mean 14.7 parts of air to 1 part of fuel.

Most lawn mowers will typically have a lever to sway choke; advanced engines now offer a thermostatically controlled automatic choke system. So if you have an engine (auto-choke) with a hot start problem, go ahead and change the Carburetor. It is good to check the choke linkages, thermostat if exits, and clean the choke and air filters. If any of the elements has gone wrong, then the engine will stop. So, it is better to replace the faulty components if cleaning is not successful.

Low oil level

Engine components like coils and plugs will stall when they are above the optimum temperature limit, and the low rise in engine temperature is observed when the oil level is low. Also, an initial start or push cannot be generated if the engine has a critically low oil level to protect the engine because it serves as a basis of coolant and lubricant in the engine. So, whenever you fill the gasoline, make sure you have checked the oil level.

Overheated Engine

Another frequent cause of motor engines not starting when hot is the overheated engine. The lawnmower engine may look small, but it generates a tremendous amount of heat when in operation. To dissipate such extreme heat, it has cooling fins. The sole purpose of the fins is to scatter the inoperable heat. However, when you mow the lawn, the grass clippings and other debris may clog or block the fins. These fins jammed with old grass stuff and debris for an extended period behaves like an insulation blanket, in return overheating the mower.

It will eventually stop running, and you will have to wait until the engine cools. The shroud or plastic housing can also trap enough dirt, debris, twigs, and clogs that will eventually lead to the engine shut off. Therefore, cleaning the fins and plastic covers once it is cool prevents the engine from stalling. What you have to do is take away the outer casing of plastic and eradicate the twigs and old grass with compressed air; otherwise, they become the prime source of holding heat close to the engine. When the engine cools, fire it up again, but it perhaps will bog down and stop once more after it overheats.

Air leaks

Loose bolts are commonly the source in a mower engine for shutting down when hot. Let me explain how. Loose bolts are source of air leaks that force an engine to sucks in too much outside air, which abandon the appropriate gas-to-air ratio. When the engine gradually heats up, the remote units expand, and all gaps become larger. Ultimately, you came across the hard running of the engine that will sooner or later shut off.

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Lawn Mower Compression Test

Therefore, periodically you have to tighten the bolts that keep the motor frame and engine components together to avoid this problem; otherwise, these bolts will probably lose after their overtime use and make the engine of your mower shut off. Check and tighten the bolts, especially those that are linked with the air intake to the engine block. An important tip to check at times the bolts that secure the whole lawnmower frame.

Problematic Compression

The compression issue is prevalent for mowers. It is mostly due to faulty valves. There is an easy way to figure out whether your mower is suffering from low Compression or not. If the mower’s starter rope seems to move easier than it used to be, it indicates the Compression is inadequate along with too large valve clearances, which is the foundation for another problem aggravated by a hot engine. If you do not know about mechanical components, you need to call a professional small engine repair person.

You can refer to a video as attached in this URL that explains the entire process:

Additional causes when your lawn mower does not start when it is hot:

Apart from the earlier mentioned reasons there are two additional reasons that can lead to your mower not, pr difficult starting when it is hot.

Presence of Alcohol in oil.

Sometimes, alcohol in oil may be responsible for shut off. Many manufacturers recommend regular gas or e10 ethanol. As E15 and e85, when set on fire, dissipate much heat so, it will damage the engine because of the low boiling point. And it voids your warranty. You can solve the problem by merely placing a riser or non-metallic shim between the engine and the carb.

Troublesome ignition coil

If your Lawnmower is new, though it is rare but possible, the ignition coil may have gone wrong. But if the mower is 10 years old, the chance is high. The ignition coil and condenser control the flow of electricity to the spark plug. They must be in the best state, and if they are not, they should be replaced on a priority basis; otherwise, it can cause severe problems in starting the engine.

Frequently Asked Questions:

Starter does not crank when hot, what to do?

This issue is the case of heat soaked. It is merely a matter of absorbing and retaining heat from under-hood sources, for instance, from the exhaust. This heat adds up the resistance in the starter’s electrical component, which dramatically reduces the current flow. The limited current in the starter results in improper cranking. Diagnose the battery, cables, and starter before the operation.

Why Honda lawnmower won’t start?

First of all, inspect the spark plug to see if it is damaged by using a spark plug tester. When the engine is cranking, if there is a vital spark, then the spark plug terminals are not defective and vice versa. Replace the spark plug if it’s defective.

A clogged carburetor can restrict the engine from starting, so replace it if it’s not working after cleaning. Insufficient or old fuel (gasoline) may be one reason for carburetor clogging; some components may evaporate with time leaving behind a stickier fluid that causes clogging. Old fuel also causes clogging of the fuel filter. You should check the ignition coil with an ignition coil tester for this issue, replace it if defective.

Torro lawnmower is not starting; what to do?

You should do a thorough cleaning of the mower if your Torro lawnmower is not starting. Do clean your mower’s Carburetor, and at the bottom of the float valve, there is a bolt; pull it out. Clean the tiny holes in that jet and make it sure by passing a wire through the holes. Also, clean the very tiny hole at the brass bolt’s top, which often gets clogged.

Final Remarks:

Indeed, a malfunctioning mower can be a significant source of distress. Being impatient in this case will not help you at all. Follow the systematical approach to solve the problem by checking every possibility mentioned above. The troubleshooting in the steps, as mentioned above, thoroughly takes you through all the issues. over, you will be furnished with the knowledge of what a repair person may need to resolve.

What Happens If You Run A Lawn Mower Without Oil

It’s common knowledge that the moving parts of a machine need oil for lubrication to function effectively. However, you could find yourself in odd situations where oil gets depleted in your lawn mower while in use.

Sometimes this will happen without your knowledge catching you by surprise. Read the rest of the article to know what happens if you run a lawn mower without oil.

Can A Lawn Mower Engine Run Without Oil?

No, a lawn mower engine cannot run without oil. If you try to run it this way, you’re effectively killing it by damaging the engine parts beyond repair.

Whether your lawn mower engine is two or four-stroke, it requires oil during operation to smoothen the moving parts such as the crankshaft, the piston, and metal bearings.

Also, with oil, there is less friction— the temperature of the internal engine components remains at normal levels preventing the engine from overheating.


Several mechanical issues could arise if your lawn mower engine runs without oil. Typical signs include:

Other signs include loud knocking or rumbling sounds coming from the engine.

Your lawn mower engine will seize if it runs without oil longer, ultimately failing. The major culprit is the increased friction leading to overheating.

Substantial damage occurs to the metal parts causing small pieces to chip away. The other problem you’re staring at is poor lubrication leading to the crankshaft and pistons interlocking with other metal parts.

Additionally, you have to deal with the premature wearing of the piston rings, potentially resulting in low compression levels.

How Long Will A Lawn Mower Engine Run Without Oil?

As per various manufacturers, a lawn mower engine shouldn’t operate without oil. However, there could be scenarios where your engine runs short of oil during operation.

Different lawn mowers vary in their reaction to an oil shortage.

For the record, a two-minute oil shortage during use is enough to cause permanent damage to the engine. However, this is not cast in stone since some engines may still have room for repairs.

The parts that take the most beating with visible damage include the crankshaft, the piston, and the bearings.

How Do You Know When A Lawn Mower Needs Oil?

As already highlighted, oil is a vital component of your lawn mower engine to ensure its functionally operational.

However, too much oil is equally bad for your engine and is a major cause of leaks and hydro locking.

You should check the oil in your mower when the mower is parked on a level surface, and the engine is off.

The dipstick is usually found on the side of the engine. It has two marks; the top mark indicates a full oil level while the bottom mark indicates low oil levels.

The oil is at critically low levels below the lower mark.

Tip: Always turn the dipstick anticlockwise when removing it from the engine.

How To Correctly Add Oil To The Lawn Mower?

Once you’ve checked the oil levels in your engine and determined that the amount is insufficient, proceed with the steps below.

Given the small nature of some dipstick tubes, you may need to use a funnel for proper filling.

Add a little oil as you recheck the oil level. Keep adding and rechecking until the upper mark is reached.

Avoid the temptation to overfill beyond the upper mark.

What Should You Do If You Accidentally Start Your Lawn Mower Without Oil?

If you accidentally start your mower without oil, the first thing to do is to stop it immediately. Look at the below scenarios to assess the damage caused:

Scenario 1: The piston is still moving slowly.

There is a considerable chance your mower engine can roar back to life.

Scenario 2: The pistons and other parts are welded together.

Newer Toro push mower with NO compressionEasy fix!

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There is little you can do about it.

If the former situation occurs, check the user’s manual for guidance on how to fix it. If that doesn’t seem to work out, you’re better off enlisting the services of a technician.

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