How to Diagnose and Treat a Lawn Mower That Won't Start

Updated on August 13, 2018
This guide will help you figure out what's wrong with your lawn mower.
This guide will help you figure out what's wrong with your lawn mower. | Source

Even if you take proper care of your mower by winterizing it and giving it regular tune-ups, it's eventually going to break down at some point in time. It's just the way of machinery: it wears out and wears down, then it needs fixing.

Hopefully you're handy with a socket and wrench, because most problems are fairly simple and straightforward on a mower.

Note: Keep in mind that every mower is not exactly the same. The newer mowers are going to have features that the older mowers don't have, such as a push button start instead of the old pull string. The mowers I've worked on range from the 1980s through 2010, and they've all been basically the same. This is a wide range of years, but you can't beat a good model, so mowers have generally remained the same.

Diagnosis of a No-Start Mower

When my mower is on the fritz, I go through a process of checking each vital engine part. The following are suggested components to check for when you have a no-start situation. Here is a brief description for each component check:

  • First check that there is indeed fuel in the gas tank. Is the fuel old?
  • If there is fuel, remove the air filter and pump the primer bulb a couple of times to make sure fuel is squirting into the carburetor.
  • Give the starter a push or a pull (depending on your mowers starting system) a couple of times. See if the fuel you just squirted with the primer bulb did anything for it.
  • Still no start? Pull the spark plug. How does it look? Is the tip fresh, with nice right angles to the tip? If the tip is rounded or broken, replace the spark plug. If it has grime or build up on it, you can try cleaning it with sandpaper. If the tip is pressed down onto the electrode, there's a serious engine problem that needs rebuilding.
  • Now snap the spark plug into the spark plug wire, but don't insert it into the engine. Set it near the engine, but don't let the top electrode touch the engine. Give the starter a push or a pull again. Did you see an electric arch go from the spark plug to the engine? If not, try again and watch the spark plug closely. It can be difficult to see in the sunlight. If there's definitely no spark, check the wires from the spark plug to the coil. If you've replaced/cleaned the spark plug, check the spark plug wire from the plug to the coil and check the next item.
  • This is where that sneaky problem comes in: the Safety Switch Setup. This deserves further explanation below.

Diagnosis and treatment for a no-start mower.
Diagnosis and treatment for a no-start mower. | Source

How to Access the Safety Switch

Click thumbnail to view full-size
Safety switch.Remove the cover. Screw A only needs to be loosened on this model.  The B screw must be completely removed.Remove the cover.Set the cover somewhere safe. The cover protects it from getting really mucked up.
Safety switch.
Safety switch. | Source
Remove the cover. Screw A only needs to be loosened on this model.  The B screw must be completely removed.
Remove the cover. Screw A only needs to be loosened on this model. The B screw must be completely removed. | Source
Remove the cover.
Remove the cover. | Source
Set the cover somewhere safe. The cover protects it from getting really mucked up.
Set the cover somewhere safe. The cover protects it from getting really mucked up. | Source

What Is the Lawn Mower Safety Switch?

The safety switch is often overlooked when diagnosing a no-start. It's the sneaky culprit, because the wear on it is insidious. It's not a regular maintenance item, so a couple of mowing seasons down the road, it's multiple parts start to break down.

The safety switch is usually inside the small box at the back of the mower engine. (See photo series below.) The switch is disengaged when the safety handle at the handle bar is pulled. When the bar is released, the safety switch engages. This causes a ground that, in turn, causes the coil to stop producing energy to the spark plug. This kills the engine. Also, when engaged or released, a spring-loaded metal strap with a rubber lining presses against the flywheel, acting as a brake helping to stop the engine from turning. (See bottom photo.)

The safety switch consists of four main components starting at the handle bar:

  • The safety bar at the handle bar must be pulled to allow the engine to start.
  • This bar is attached to a cable that runs down to the safety switch itself.
  • The safety switch is attached to a "brake." This brake helps slow the flywheel.
  • The brake has a pivot on the far end of it. When the safety switch is engaged, the brake's pivot disengages the ground to the coil.

Safety Switch Four-Component Inspection

Let's go over the safety switch four-point system.

1. Part of the sneaky culprit system is the cable that runs from the handle to the safety switch. These cables are vulnerable to:

  • Weather: Rain and snow cause water to seep into the cable and rust.
  • Usage: This wire is pulled and held taut every time the mower is started, until it is released and the mower shuts down. That's some heavy usage.
  • Stretching: Cables stretch. This cable stretches just like every cable. But when this cable stretches too far, then when the safety handle is pulled the brake does not disengage, and the coil remains grounded. That's a big no-start situation!

Pick up a new cable at the hardware store or lawn store. They're not expensive. I've also used a bicycle cable when I was in a pinch. The brake cable on a ten-speed is proper length and easy to modify. Either way, replace this cable, and you may find your troubles are gone and your mower starts!

2. Move down to the safety switch itself. Remove the cover. (See above photos.) Debris, sticks, twigs, etc. can get up under the box and break or dislodge the springs. Are the springs dangling or missing? Does the safety switch have smooth operation when the safety bar is pulled?

3. Watch the brake as you activate the safety bar. Does it move away from the flywheel, just a little bit? (See bottom photo.)

4. The last part of this safety system is the kill wire or coil ground. Check the kill wire. The kill wire travels along an active area of the lawn mower. Grass, branches, debris, and motor vibration can all cause abrasions on the wire or rip the wire off. Follow the wire and make sure it is still attached to the coil with no breaks, or grounds, on it.

Look closely to see the gap created between the brake and the flywheel when the safety bar is pulled.
Look closely to see the gap created between the brake and the flywheel when the safety bar is pulled. | Source

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