It’s general knowledge that to make fire, you need oxygen, a heat source, and fuel. Well, in a gasoline internal combustion engine, oxygen comes from the air, the fuel is gasoline, and the source of heat comes from spark plugs. That means spark plugs play an integral role in how the engine runs and whether or not it runs at all.
So, it doesn’t surprise that old spark plugs can cause issues, but problems after changing them are also fairly common. Those problems include misfires, difficulty or inability to start the engine, and one or more cylinders not firing at all.
Most Common Problems After Changing Spark Plugs
A couple of things we should mention about the following problems before we cover them in more detail. The first is about misfires after replacing spark plugs. Now, if your car was misfiring before you replaced spark plugs, then something else could be causing the misfires, like faulty ignition coils or spark plug leads.
The same goes for the remaining two problems. Namely, the car won’t start or is running on two or three cylinders. In case it was doing that prior to new spark plugs, you should inspect other ignition system components too. But if the car runs worse after changing spark plugs, here is what might be happening.
Car Misfires After Replacing Spark Plugs
In case your car had only begun misfiring after you replaced the spark plugs, chances are the new spark plugs are damaged. You might also notice difficulty starting the car after changing spark plugs if it has developed misfires. Also, you may have bought the wrong type of spark plug with an incorrect gap, heat rating, or material.
Every engine needs a specific spark plug type to work properly, and even though most spark plugs will fit, that doesn’t mean they are the correct ones. And in case you installed used spark plugs, which you should never do, you can be sure that they are faulty.
Car Won’t Start After Changing Spark Plugs
A non-start condition is another common problem after replacing the spark plugs. Luckily this seldom happens because of defective plugs or other faulty components but rather because they weren’t properly installed. To be specific, the spark plug leads are not plugged into corresponding spark plugs, or you forgot to plug something in, like an ignition coil harness.
Also, the engine might not start if you bought the wrong type of spark plugs; however, that’s unlikely because even with the wrong spark plugs, the engine will at least fire up. And while defective spark plugs are also a possibility, the chances of all the new spark plugs being defective are slim to none.
One or More Cylinders Is Not Working After Changing Spark Plugs
If one or more cylinders are not working, it’s still technically considered a misfire. However, a random or frequent misfire sounds a lot different than a non-functional cylinder, and it’s a separate problem altogether.
An engine with one or more cylinders that are not firing sounds like a lawn mower; the engine shakes and vibrates, the performance and fuel economy are significantly worse, and a flashing check engine light plus a rough idle after spark plug change.
Solving this problem, provided the engine was running fine before the new spark plugs, usually consists of checking if everything is plugged in properly. Other than that, you might be dealing with one or more defective spark plugs.
Car Stuttering, Shaking, and Stalling After Changing Spark Plugs
This one comes from my own experience. I had a case where a customer brought in his car because it was heavily misfiring and stalling. The car would sometimes refuse to start, and other times it would fire up but run rough and stall after a couple of minutes.
The customer mentioned that the problems began shortly after he replaced the spark plugs. The spark plugs were in fact new, and they were the correct type. The symptoms were really peculiar since spark plugs will either work or not. To see how we solved the problem, scroll down to “Fixes.”
Why Your Car Makes Problems After Changing Spark Plugs
The main reasons why your car makes problems after changing the spark plugs include incorrect installation, defective spark plugs, non Branded Spark plugs, or the wrong spark plug type. Now, since we already explained all the common problems after replacing spark plugs and what causes each of them, it’s best if we now explain how to fix those problems. So, here goes.
Engine Misfiring After Changing Spark Plugs – Fixes
In case your car is running a little rough or the OBD2 scanner shows a P0300 (random misfire) code, the first thing you should do is check what spark plugs your engine requires. You can find that information in your owner’s manual, sometimes on a sticker in the engine bay, or you can ask an auto parts store clerk to check for you. Then, if it’s needed, replace the spark plugs with the correct ones.
Also, check whether or not the spark plugs you are buying are pre-gapped, which in most cases, they are. But if you drive an older car, say from the 80s or older, you might need to gap them to the correct specification. And lastly, if you bought used spark plugs or only replaced one of them, get a full set of new spark plugs; otherwise, the engine will always run rough.
Car Won’t Start After Changing Spark Plugs – Fixes
The number one reason why a car doesn’t start after changing the spark plugs is incorrect installation. And by that, we mean improperly plugged spark plug leads, ignition coil connectors, or something else isn’t plugged in.
It’s important to know that every spark plug lead corresponds to a specific spark plug, and the same goes for ignition coil connectors in coil-on-plug systems. By plugging the number two lead into a number three spark plug, neither of the two cylinders will work. Then, do that with the rest of them, and the engine won’t start because all the spark plugs are firing at the wrong time.
To fix that, look for diagrams online for your specific engine to see which spark plug lead goes to which cylinder. Also, if you didn’t unplug the leads from the ignition coil, you can go by spark plug lead length. The longest spark plug lead goes to the spark plug furthest away from the ignition coils, and so on.
Also, in case you have to unplug something else to reach the spark plugs, like a MAF sensor, a camshaft position sensor, etc. Recheck if it’s plugged in properly.
One or More Cylinders Is Not Working After Changing Spark Plugs – Fixes
In case only one of the cylinders isn’t firing after changing the spark plugs, and you get a specific cylinder misfire code (P0301-P0312), chances are you got a defective spark plug. To check if that’s the case, switch the spark plugs around and see if the fault code moves to the cylinder with the potentially faulty spark plug. And if the answer is positive, buy a new one.
Otherwise, make sure everything is plugged in properly and that there is no damage on any of the wiring or spark plug leads. On the other hand, if you get two cylinder-specific misfire codes, try switching the spark plug leads between the two and see if that helps. If not, do the same with spark plugs, and if the misfire moves with them, they are defective.
What Will Happen If You Replace the Wrong Spark Plug
First and foremost, you should always avoid replacing only one spark plug. Luckily, spark plugs are cheap, and a full set for a four-cylinder engine rarely costs more than 15 bucks. But if you have to, nothing will happen if you replace the wrong one other than the fact you won’t solve whichever problem the engine had earlier.
The best way to avoid that is to use an OBD2 scan tool, so you know exactly what the problem is and on which cylinder. Otherwise, trial and error is the only method you can use until you replace the correct one. But again, you won’t cause any immediate damage by replacing the wrong spark plug.
FAQs
Q: Why is my car misfiring after changing spark plugs?
If your car is misfiring after changing spark plugs, chances are you bought the wrong type of spark plugs. That usually happens if you buy spark plugs without visiting an auto parts store. On the other hand, you might have mixed up the spark plug leads, but in that case, at least two cylinders wouldn’t be firing at all.
Q: How do you know if spark plugs are installed wrong?
The way to install spark plugs wrong is to cross-thread them, but as long as they are tightened, there shouldn’t be any problems until you try to remove them. Other than that, you might notice that the engine is shaking if you mix up the spark plug leads or ignition coil connectors.
Q: Why is my car shaking at idle after a spark plug change?
Your car is shaking at idle after a spark plug change, most likely because it’s misfiring or because one of the cylinders isn’t working. If you notice shaking only at idle, you probably bought the wrong type of spark plug. But if the shaking continues while driving, you may have switched around the spark plug leads.
Q: Should I replace ignition coils with spark plugs?
No, there is no need to replace ignition coils together with spark plugs. Ignition coils don’t have a set service schedule regardless of the type of coil and are only replaced when they fail. But of course, you won’t cause any damage if you do replace them.
Q: Is a torque wrench required for spark plugs?
Technically yes, a torque wrench is required for spark plugs to avoid leaving them loose or overtightening them and potentially ripping the cylinder head threads. But truth be told, not that many people use torque wrenches on spark plugs, and you just tighten them by feel.
Final Words
Ultimately, the most common problems after changing spark plugs occur when the same problem was already present, and changing the spark plugs didn’t solve it. Of course, that means the spark plugs were not the issue and are not the problem now.
However, if your car ran fine before you replaced them and it started misfiring, refusing to start, or shaking violently because one of the cylinders isn’t firing, then it’s either defective spark plugs or wrong installation. To solve those problems, first, make sure the spark plugs are the correct ones for your vehicle. If that checks out, maybe you mixed the spark plug leads or ignition coil connectors, and lastly, you might be dealing with one or more defective spark plugs.
