Why Won't the Spark Plug Fire on a Lawn Mower and How to Fix It

Dealing with a lawn mower that won’t fire can be exasperating. Whether it's an old Snapper push mower or a newer Troy-Bilt riding mower, there are a few common reasons and solutions you can consider. Here, we'll explore the most likely culprits and walk you through how to fix the issue.

Common Causes of a Non-Firing Spark Plug

One of the most common reasons why a spark plug won’t fire is because the ignition system has run its course. Over time, the spark plug can lose its ability to produce an effective spark. This could be due to age or wear and tear. Additionally, the insulation around the spark plug may become damaged or corroded, leading to poor conductivity and, ultimately, no spark.

The Ignition Coil: An Essential Component

Another frequent cause of a non-firing spark plug is a faulty ignition coil. The ignition coil is a critical component that transfers the low voltage from the battery to the high voltage needed to ignite the fuel in the engine. In the case of Briggs and Stratton engines, this component is often referred to as a 'magnetron ignition.'

I’ve encountered this issue on both my 25-year-old Snapper push mower and my 10-year-old Troy-Bilt riding mower. Both are equipped with Briggs and Stratton engines. Upon replacement, the issue was resolved, and the lawn mower was back in full working order.

Replacing the Ignition Coil

To replace the ignition coil, you’ll need to ensure that the part either matches or is an equivalent to the original. The appearance of the coils can vary, so finding the right one is essential. Follow these steps to replace the coil:

Identify the part number on your old ignition coil or the model information from your engine. The engine model can often be found on a decal on the engine or embossed on the valve cover.

Remove the main fan shroud from the engine. The fan shroud is the central cover, typically red in the middle of your engine. If you have a pull start, it will be integrated into the fan shroud, and both the cover and pull starter will come off as one unit. For engines with an additional trim cover integrating the fuel tank and air cleaner, remove the trim cover and fuel tank trim first.

Once you have access to the ignition coil, simply replace it with the new one. The process is straightforward, and there are many YouTube videos available to guide you through the process.

Pictures for Reference

Below are some visuals to help guide you through the process. The rusty piece with the coil and spark plug lead sitting next to the flywheel/cooling fan is the ignition coil unit. The appearance of the coils may vary, so finding one that matches or is an equivalent is important.

Additional Tips

Here are some additional tips to ensure that your lawn mower’s ignition system is in optimal condition:

Regularly check the spark plug and clean or replace it as needed to maintain optimal performance.

Inspect the wiring for any signs of damage or wear and make repairs as necessary.

Do a visual inspection of the ignition coil. If it’s cracked, damaged, or visibly worn, it’s time to replace it.

Conclusion

When your lawn mower won’t fire, it’s essential to troubleshoot the ignition system. The ignition coil is often the culprit, and replacing it can be a simple and effective solution. By following the steps outlined above and using the guidance provided, you can restore your lawn mower to working order quickly and efficiently.