How To Tell If A Lawn Mower Spark Plug Is Bad


People change spark plugs in small engine equipment like way more up and  than they need to. They sell them in department stores, 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 small engine every year, especially on something like a rototiller were you just abused the heck out of it for four hours and then put it away. So what I’ve got then in this next post is how to tell if a spark plug is actually any good or not.

spark plug

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.

Maybe it’s my Scottish background from way back five generations ago but I have a hard time separating with my money and it really drives me crazy that people would go to  fix their own machines and they’re trying to save a buck to. are essentially ripped off by replacing a spark plug every year. A lawn mower might  have 50 hours on it at the end of the year and that’s not a lot so anyway that’s my opinion.
Related Post: /best-corded-electric-lawn-mower-reviews/

How to do:

Cleaning up

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’s okay to keep using them, especially they’re really good RJ 19 LMS. They are well made heavy plug and they can go a long time.

Now we’re gonna do spark plugs 101. We have two spark plugs here: one is labeled bad and the other one is just a normal good plug. Neither one of these plugs are the resistor model so we’ll talk about that in a minute. So first of all we’ll have a good look at a good spark plug and my meter.

Setting the resistance:

We have particular resistance measurements and with the leads closed we get point one or point two ohms and that’s the resistance of the leads. So on this good little plug, we’re going to take a resistance measurement of the length of the electrode inside the spark plug. Right there if we touch on that silver spot, this is hard to do that blood 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. We’ll talk about that in a minute so 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.

So 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.


I’ll cut across here to this 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 right here.

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 bit because it’s a bad blood.

There we go we have 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 blood. There it is now it’s even less. Sometimes it’ll pick up your body map but you can measure your body’s resistance and getting it mixed up.

I’m not touching anything on this end here so there we go. Six million ohms from the ground to the point. Now it changes a bit.. see but 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

Here is a video about Checking, Cleaning & Replacing Lawn Mower Spark Plug:

How To Tell If A Lawn Mower Spark Plug Is Bad