It can be very frustrating to find your lawn mower doesn’t start when you need it most. The grass isn’t going to just stop growing because your mower is down. Many items that prevent a diesel mower from starting are problems that can be troubleshooted and repaired by most lawn mower owners.

A diesel lawn mower won’t start when there is a bad or insufficient fuel in the fuel tank; plugged air filter; clogged fuel lines; plugged fuel filter; faulty fuel pump; bad glow plug; bad battery; blown fuse; bad starter solenoid; or failed switch.

Keep reading for additional items that can cause a diesel mower to not start. Before working on your diesel mower, follow all safety procedures outlined in your operator’s manual to prevent injury.

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Follow all safety instructions provided in your equipment operator’s manual prior to diagnosing, repairing, or operating.Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

15 Reasons a Diesel Lawn Mower Won’t Start

No Fuel in the Fuel Tank Will Cause a Diesel Mower Not to Start

Of course not having diesel fuel will cause your mower not to start. Sometimes, out of frustration, you forget to check the simple items like fuel in the tank. This is why I mention it.

Solution: Let your engine cool for a few minutes if it is hot. Fill your lawn mower tank with diesel fuel. Bleed air from the fuel system by allowing it to run for about a minute if you ran out of fuel.

Bad or Old Diesel Fuel Will Cause a Diesel Mower Not to Start

It is best to use diesel fuel within 6 months of purchase. Leaving diesel in your mower for a long period can result in sediment and gummy solutions forming from exposure to oxygen and water.

Signs your diesel fuel is getting old are the darkening of the fuel, increased sediment in the fuel, filter clogging more often than normal and the fuel filter appearing dark in color.

Because it is important to run good clean diesel in your lawn mower, it is best to purchase diesel from a busy fuel station. Not only does it need to be busy, but it also needs to be busy with diesel fuel users.

I found this out the hard way. I always purchased my diesel fuel at a busy well-known fuel station in the area.

However, I didn’t pay attention to the type of vehicles at the pump as they were mostly fueled by gas. It wasn’t until I was experiencing running issues that I checked my fuel filter and found it was black.

Diesel sitting in storage tanks at a fuel station will develop a sludge in the tanks when fuel has been sitting for a long time.

Solution: Remove old diesel fuel and fill the tank with fresh fuel. If your mower still does not start, continue to check the fuel components for restrictions or failures due to running dirty old fuel.

Plugged Fuel Filter Will Cause a Diesel Lawn Mower Not to Start

The fuel filter’s function is to strain fuel coming from the fuel tank to prevent dirt and sediment from entering the fuel system and engine. A plugged fuel filter will prevent fuel from passing through the filter.

Solution: Replace a plugged fuel filter. You will find a small arrow on the side of the filter. The filter must be installed with the arrow pointed in the direction of your fuel flow.

If you find the filter is very dark in appearance because of old fuel, you should drain your fuel tank and refill it with fresh diesel. Start your mower and allow it to run for a minute to bleed air from the fuel system.

Blocked Fuel Line Will Cause a Diesel Lawn Mower Not to Start

A lawn mower fuel line can become clogged by sediment and gummy deposits preventing fuel from getting to the engine.

Using the fuel shut-off valve to start and stop fuel flow, check each section of the fuel line to ensure there is good flow. When you find a restricted fuel line proceed with the following steps:

Solution: Remove the fuel line from the mower, spray carburetor cleaner into the tube to loosen the clog, and use compressed air to blow air through the tube to remove the restriction. Repeat as necessary.

Replace the fuel line with a new line when you can’t remove the clog. Start your mower and allow the mower to run for up to a minute to bleed the fuel system.

Water or Air in the Fuel System Will Cause a Diesel Lawn Mower Not to Start

When you have excessive water in the fuel tank, water sinks to the bottom of the tank and can cause starting problems. It’s best to drain the fuel tank and refill it with fresh diesel fuel. Air trapped in the fuel system can also have a negative effect on your tractors starting and running.

Solution: Start your mower and allow it to run for a minute to remove air and water from the fuel system. You may want to add a product like Sea Foam to help clean the fuel system and reduce moisture.

Bad Fuel Cap Will Cause a Diesel Lawn Mower Not to Start

Without a vent in the fuel cap, the tank will act like a vacuum and restrict fuel from flowing through the fuel lines. Old fuel can cause clogging in the vent so air will no longer pass through the cap.

To isolate your cap as being the cause of your lawn mower starting problem, start and run your mower for a while with the cap off and then with it on to see if your cap affects the running ability of your lawn mower.

Solution: Replace your diesel fuel cap.

Bad Glow Plugs Will Cause a Diesel Lawn Mower Not to Start

Glow plugs assist with heating a cold engine. The glow plug electrifies a heating element found at the tip of the plug. The hot glow plug tip compressed air in the cylinder and injected fuel to form combustion.

Solution: Test your glow plugs and replace them if they are no longer working. Read more about different methods to test your glow plugs in this guide from General Technologies Corp.

Plugged Air Filter Will Cause a Diesel Lawn Mower Not to Start

A lawn mower requires clean air to run. The air filter prevents dirt and debris from entering the air intake and contaminating the engine. Dirt in the engine can significantly damage an engine.

Never run your mower without an air filter even if it’s only for a short period of time while you source a new filter.

When an air filter gets plugged in so air is no longer able to pass through the filter, your mower won’t start. It is important to regularly check the air filter and keep it clean.

By checking, cleaning, and replacing this inexpensive part when needed, you can prevent an expensive engine repair.

Solution: Follow these procedures for cleaning a paper air filter:

• Remove the air filter from the housing carefully. You don’t want to knock any dirt or debris into the air intake.
• Wipe out any dirt that is remaining in the housing.
• Knock out the excess dirt in the filter by tapping it against a solid surface.
• Hold the filter up to a light source and check for light shining through the paper element.
• Reuse the filter if you can see light pass through the paper. If you cannot, replace your filter with a new air filter.
• Install the air filter and attach the cover.

Faulty Fuel Pump Will Cause a Diesel Lawn Mower Not to Start

The fuel pump can fail from the usage of old fuel. Sediment, dirt, and water can cause the fuel pump to deteriorate and fail to work.

Solution: To identify a failing fuel pump, start by making sure you are getting good fuel flow to the pump. This includes making sure fuel is passing through the fuel filter and fuel lines. You don’t want restrictions on the fuel lines caused by blockages or kinks in the lines.

Next, refer to your mower operator’s manual to get the correct pressure reading for the fuel pump. Using a pressure gauge, attach it to the pressure hose coming out of the fuel pump. When the pressure reading is lower than the manufacturer’s specification, it needs to be replaced.

Bad Battery or Loose Terminals Will Cause a Diesel Lawn Mower Not to Start

Diesel lawn mowers require a battery to start. Make sure your cables and battery terminals are secure.

Clean any corrosion you find on your terminals using a baking soda solution (2 cups water to 3 heaping tablespoons of baking soda). Once you confirm you have a good connection, continue testing the battery.

Solution: Test your battery with a multimeter. You want a reading at about 12.7 volts. Charge your battery if it is lower than this level. You can find more information on charging your battery here. A battery that is dead or won’t hold a charge must be replaced.

Bad Starter Solenoid Will Cause a Diesel Lawn Mower Not to Start

A lawn mower solenoid on your lawn mower is an electromagnetic switch that is like an on-off switch that actuates the starter motor to turn over the engine.

A click or hum when turning your ignition key is an indication to check your solenoid. Another indication your lawn mower solenoid may be bad is when a wire attached to your solenoid gets hot and begins to smoke or melt.

Solution: Test your lawn mower solenoid by following the steps here. Replace your solenoid if it is found to be bad.

Blown Fuse Will Cause a Diesel Lawn Mower Not to Start

Fuses are installed to protect your diesel lawn mower’s electrical system. Check your mower to make sure you don’t have a blown fuse. The location of the fuse block can vary depending on the manufacturer.

A good place to start is looking around your battery. It may be placed under or next to this area. You may also find the fuse behind your dash.

Solution: Replace a blown fuse. Use a fuse with the same amperage as the fuse you are replacing. If you continue to blow fuses, you should bring the lawn mower to your local repair shop or dealership to troubleshoot the root cause of the electrical failure.

Bad Safety Switch Will Cause a Diesel Lawn Mower Not to Start

Your diesel lawn mower may use several safety switches in its operator’s presence control system. The switches are designed to kill the engine when the operator leaves the seat and the mower deck is engaged.

It also allows the mower to start when the parking brake is set and the PTO is disengaged. A faulty switch may not recognize the operator is in the seat or the parking brake is engaged.

Refer to your operator’s manual for the different types of safety switches used in your type of lawn mower along with the procedures to test them.

Solution: You can temporarily bypass the safety switch to identify a bad switch. Do not operate a mower without the safety switch installed for your safety. Always have safety switches installed and working on your equipment. Replace a bad switch. 

Faulty Charging System Will Cause a Diesel Lawn Mower Not to Start

If you have confirmed your wires are secure and the battery is charged, but you continually run into a problem where your diesel lawn mower won’t start until you charge the battery each time you use it, you should check the charging system.

A faulty charging system on your lawn mower will drain the battery and not keep it charged causing your mower not to start.

A bad stator, alternator, or several other electrical parts can be the cause of your starting problem. Read this article to test your charging system here using an ohm meter. 

Solution: I recommend having your lawn mower looked at by a professional lawn mower dealership to identify your charging system. There are so many pricey electrical components that can cause your starting problem.

If you are not experienced with the charging system, you will probably just be throwing parts at your mower hoping it will solve the problem.

This can get very expensive especially when most stores do not allow you to return electrical components if they aren’t the cause of your problem.

Make sure the mechanic is experienced in this area or bring it to your local dealership to be fixed. I have seen, on many occasions, mechanics just throwing parts at mowers, billing the customer, and never fixing the problem.

Those customers ended up bringing their mowers to the dealership to be fixed correctly. Most top-brand mowers require their dealerships to have factory-trained mechanics on staff.

Incorrect Operating Procedure Will Cause a Diesel Lawn Mower Not to Start

Your lawn mower has safety features that won’t allow your mower to start unless you follow its starting procedures.

This may be setting the brake, putting the mower in neutral, or waiting for the glow plugs to warm the engine. Starting procedures vary depending on the type of lawn mower you are using.

Solution: Refer to your diesel lawn mower’s operating manual to ensure you are operating your lawn mower correctly, so you don’t set off the safety features that shut off your lawn mower or don’t allow it to start.

Still Having Starting Problem with Your Diesel Lawn Mower?

If you are still having problems after going through this list or you don’t want to tackle some of the more complicated troubleshooting steps, have your local lawn mower dealer or repair shop do the work.

I recommend taking your mower to an experienced mechanic when you are not comfortable or knowledgeable about working with electrical systems and engine repairs to prevent additional damage to the mower or injury to yourself.