What Are the Signs That a Spark Plug Is Bad?

Having trouble with an engine that won't start or runs rough? A bad spark plug could be the culprit. Here's how to tell if you need to replace it.

Our editors and experts handpick every product we feature. We may earn a commission from your purchases.

Spark plugs are small, cylindrical devices that use electricity to trigger combustion of the gas/air mixture in engines, allowing them to start and continue running. They do this by turning incoming electrical voltage into a spark inside the cylinder or cylinders of an engine, igniting the pressurized fuel inside. This ignition forces the piston downward, generating power for the engine. The piston is pushed back up again by exhaust, and then the cycle repeats.

Like most engine parts, spark plugs can fail for a few reasons. This guide will focus on small engines in tools like lawn mowers, rototillers, snow blowers, chainsaws, etc. Although spark plugs for small engines look like those for road vehicles, they’re considerably more likely to fail in small engines for several reasons. Here’s what to look for when you suspect spark plug failure.

Signs of a Bad Spark Plug

Begin by trying to start your machine. If it has fuel which isn’t blocked from entering the piston anywhere (like the carburetor, if the fuel has been sitting unused in the tank for too long), it should start.

  • If the gas tank is full and the fuel is good but your small engine won’t start, you probably have a faulty spark plug.
  • If the fuel checks out and your engine starts but runs rough, a bad spark plug is likely the culprit.

Keep in mind that each cylinder in your engine has its own spark plug. Small engines can have one cylinder, two cylinders or four cylinders, depending on engine size.

What Does a Bad Spark Plug Look Like and What Makes It Look That Way?

Start by disconnecting the plug wire from your engine’s spark plug. Then use a wrench to unthread the spark plug from the cylinder head, removing it from the engine. Examine both ends of the plug carefully. With small engines, the chance of physical debris causing spark plug failure shouldn’t be ignored. The plug might have any of the following issues:

  • Black, chunky carbon buildup spanning the gap between the sparking end of the plug and the hook-shaped metal piece overhanging the end.
  • Dark carbon residue built up on the ends but not spanning the gap.
  • Oily residue on the end (most likely to happen with two-stroke engines that use a gas/oil mixture).

It’s also possible for spark plugs to fail internally, if their electrical components stop working. When this happens, there won’t be any visible signs that the plug is bad, other than its failure to spark or only producing a weak spark. If you suspect your spark plug has failed internally, use a spark plug checker to find out.

Another possible cause of failure is if the gap where the spark forms needs adjustment. The difference between a correct and an incorrect gap could be a matter of several thousandths of an inch, and will probably be too small for you to detect by eye. Refer to your machine’s owner’s manual to find out what the spark plug gap should be, then use a spark plug gap tool to measure your plug and adjust the gap, if needed.

How Does a Bad Spark Plug Sound?

There are only two possible sounds that may indicate a bad spark plug: Your engine running rough, even though it’s supplied with fresh fuel, or the silence of it not running at all.

Fixing a Bad Spark Plug

There are two ways to deal with a bad spark plug: Clean it if it’s dirty, or replace it if it’s not.

Clean: If any part of your spark plug is covered by a dark-colored buildup of carbon, use a soft wire brush to carefully clean it. Make sure all the carbon is removed from both ends. If an oily residue covers the plug, wash it in warm water and soap. Water won’t hurt a modern spark plug.

Replace: If your failed spark plug is physically damaged, or if it looks fine but isn’t sparking, simply buy a new one. Spark plugs are cheap, and there’s no easy way to repair a damaged one yourself. Make sure the new plug has the same thread pattern and gap distance as your old one.