How To Tell If A Lawn Mower Spark Plug Is Bad

People usually change spark plugs in small engine equipment way more than they need to. They sell spark plugs everywhere and yes, they follow more than the spark plug in a car. But you don’t  have to put a new spark plug in your engine every year, especially on something like a rototiller where you just abused out of it for four hours and then put it away. So this article will show you how to tell if a spark plug is actually bad or good.

Things You’ll Need

  • A wire brush. Remember to clean them up and gap them.
  • If you dont’t have a wire brush, a bench grinder with a brush on it will work well.
  • An ohm meter to check and see if that spark plug is actually okay or not.
Related Post: Top Rated Corded Electric Lawn Mowers

How To Check A Lawn Mower Spark Plug

Cleaning Up

Cleaning up a lawn mower spark plug can help us identify clearly the situation of the plug and consider whether it’s good or bad.

For example, the RJ 19 LMS. I’m probably gonna get some negative feedback on this one but I think if you clean them up and the electrode doesn’t look too grounded off, and they test okay electrically, I think that it is good to keep using them. They are well made as heavy plug and they can go a long time.

Setting The Resistance

Considering that we’re having two spark plugs: one is labeled bad and the other one is just a normal good plug. Neither one of these plugs are the resistor so first of all we’ll have a good look at a good spark plug and my meter.

We have particular resistance measurements. With the leads closed, we get point one or point two ohms and that’s the resistance of the leads. So on the good little plug, we’re going to take a resistance measurement of the length of the electrode inside the spark plug. If we touch on that silver spot, this is hard to do. That plug has four point three so very little resistance in it.

It’s a good spark plug if that was a resistor spark plug. It would measure somewhere between 10,000 and 20,000 ohms. Therefore the way to check a spark plug to see if it’s good is to check the resistance between the tip of the plug and ground.

If the spark plug starts to leak to ground, that’s what you’ll measure. And on this one, there’s no resistance to ground. We have resistance through the plug to the tip right, which is our point three but no resistance to ground and if we measure from ground to here to two. That is also the same thing it’s open but if we touch it to the ground which is all over.

spark plug


Cut across to the little the ground tip of the plug. It should be short of course so a good spark plug has no leaking to ground. Now we’re going to test the bad spark plug which is labeled bad in the aforementioned example.

Put the electrical of a millimeter on the top and now we will measure to the case of the spark plug. And we have eight makes or eight of nine mix of resistance from the top of the plug which would be leaking through to the engine block. Because this is all one piece and it screws into the engine and that is a bad spark plug, it changes a little.

It will be around nine million ohms now to a 20,000 volt spark. That’s not much resistance and it’ll leak through we can also measure it from the ground to the little electrode in plug. There it is now it’s even less.

If that’s a bad plug and they’ll mix you up, one day it’ll work and when the next day it won’t. So that is a bad sparklet one ohm between it and four to eight million ohms leaking. That’s what kills the spark.

Read more: How To Clean Lawn Mower Air Filter
How To Tell If A Lawn Mower Spark Plug Is Bad