Signs of a Bad Spark Plug

Many people don’t give a lot of thought to spark plugs, but they play an incredibly vital role in making small engines work effectively. If your spark plug isn’t working properly, you will notice a decrease in performance, and eventually, your engine may even stop running altogether. So how do spark plugs work? 

You’ll find combustion engines located inside your car or lawnmower. In these types of engines, a mixture of air and fuel enters what is called the “combustion chamber.” A piston then compresses this combustible mixture. The spark plug provides an electric current that ignites this mixture with extreme heat. This creates pressure that forces the piston out of its cylinder, thus producing power for your machine so that it can operate. 

Like many components of an engine, spark plugs can fail for various reasons. This blog will focus on the types of small engines found in equipment like lawnmowers, snowblowers, and chainsaws. Read on and learn how to spot the signs of a bad spark plug. 

Your engine is running rough or vibrating

If your engine has been running for longer than usual before it starts, your spark plug may have started failing. The same also happens when your engine is not producing enough power or lacks the power it used to have. 

What to do: If you suspect that your spark plug may be the culprit, it’s always best to take a closer look. In some cases, corrosion can build up on the tip of your spark plug, which can prevent it from firing properly. 

You notice a decrease in your engine’s performance

Has your engine slowed down considerably and is working harder than it should? This can be another sign of a faulty spark plug. You might want to consider changing your spark plugs in order to speed things up again

Your spark plugs start firing sporadically or not at all

If your engine is running, it’s important to keep an eye on how often your spark plugs are firing. If they’re not firing properly, or even at all, this could be a sign that the ground circuit isn’t working and needs to be replaced. Otherwise, this can lead to higher emissions, and your engine is likely to stall if left unchecked for too long. 


Your ability to identify a faulty spark plug is important so that you know when it’s time to replace it. But what does a bad spark plug look like?  Take a look at your spark plug. If you notice that it is wet, oily, or dirty, then you know something needs to be done. 

If you see a chunky carbon build-up that spans the gap between the sparking end of the plug and the metal piece overhanging the end, this also means your spark plug may need to be replaced. The same goes for if you see a dark carbon residue built up that doesn’t span the gap.


To change your spark plug (and save yourself from paying for expensive repairs to your engine), you’ll need to: 

  • Find an appropriate wrench – usually, it’s just one size smaller than whatever was there before. 
  • Turn off your engine so it cools down first. 
  • Remove the wires one at a time to avoid sparking nearby gas vapors (the last wire being anything connected directly grounded).
  • Loosen the nuts holding the old spark plugs until they stop turning altogether, but don’t remove them yet.
  • Push the new spark plugs into place firmly while twisting the nuts to tighten them again.
  • Replace one wire at a time before turning your engine on and testing if it is all set.

If you think there may be something wrong with your spark plug, don’t wait until things get out of hand before doing something about it. Be sure to have it inspected immediately, and replace it if necessary. 

If you need professional assistance with your spark plugs, or anything related to your small engine, visit Greg’s Small Engine. At Greg’s Small Engine, we have experience with nearly every make and model of lawnmowers and snow blowers. We provide high-quality manufacturer engine parts and maintenance services. Give us a call for more information.