Cylinder 3 Misfire after Spark Plug Change - How to Fix?

Cylinder 3 Misfire or P0303 error after the replacement of spark plugs may show up because of the bad spark plug, faulty ignition coil, or lack of compression. Also, the problem can be different like the issue with a fuel injector. It may be a coincidence that your vehicle malfunctions right after the spark plug replacement, but the ignition system should be still checked first.

P0303 code highlights

• Level of urgency:Medium
• Possible culprits:Bad plugs, bad coils, issues with fuel supply, air supply problems
• Price for repair:$100 - $550
• If neglected:Loss of power, harsh work, inefficient fuel consumption, vibrations
• DIY repairs:Possible but complicated
• Can you drive?Yes

Cylinder 3 misfire after spark plug replacement - why?

So, the most likely reason, in this case, is that the spark plug is just not working properly. Obviously, we are talking about the spark plug that is installed in the third cylinder.

Now, how do you locate the third cylinder? In an inline engine, it's easy. The first cylinder is the one that is located near the timing system cover (usually, on the passenger's side of the vehicle). Then, you just locate the third one easily.

In a V-shaped engine, one head is closer to you when you are standing in front of the engine bay. The closest cylinder is cylinder 1, then you need to just count cylinders in this head and find the third one. In a V6 it will be the last cylinder on this head while in a V8, you will have another one further.

Now, when you can locate the third cylinder, it's time to look at possible reasons for the problem:

• The spark plug is not really good. This is the number one culprit in this case because you've just replaced it and it doesn't work. It may be of bad quality or just inappropriate for this engine.
• The spark plug is over-tightened. Or it may also be not tightened enough. In this case, you will need to use a torque wrench to tighten the spark plug properly and try again.
• The ignition coil is damaged. DIY spark plug replacement may be traumatic for a coil. This is a tender part that needs careful attitude and a person with no experience can easily damage it.
• Damaged wiring that connects the ignition coil. To replace a spark plug, you will need to take off the coil. Maybe, you damaged some wire or they were damaged and now you broke a wire or a connection.
• Compression in the third cylinder. If you couldn't locate the problem, one more culprit worth checking is the compression. If the thread for the spark plug in your engine head is damaged or the head is cracked, for example, it may make the engine lose compression.

These are all possible ignition-connected problems that can cause issues with misfiring cylinder. But it doesn't mean that there couldn't be a coincidence. There are other problems that can cause engine misfire like a broken or clogged injector, problems with valves, issues in the engine block, etc. It would take me too long to describe them all here. So I'll focus exactly on ignition problems in this article just because I know you've replaced spark plugs in the engine.

So, nearly all problems are easy to check and solve but the compression problem. This issue you should leave to professionals. And in the next parts of the article, I will tell you why.

Let's see how you can check and solve the problems by yourself:

1. Spark plug check

It's quite easy to check whether the spark plug is the culprit for the cylinder 3 misfire. Just do the following:

• take off the spark plug from cylinder three and replace it with the spark plug from any other cylinder;
• put on the ignition coils and make the car ready to start;
• start the vehicle and see if the problem of misfire is still there;
• use your OBD2 scanner to see if the code is still P0303 or it has changed the last digit;
• if the code is the same, the spark plug should be OK and you can move on;
• if the code is different, you can be sure this is all about the spark plug.

It's easy and straightforward and everything you need for this is a cheap code scanner that you can buy for a couple of dollars.

2. Ignition coil check

Since there is a high chance of ignition coil damage, you can check it, too. For this, you will need to do the following;

• disconnect the ignition coil in cylinder 3;
• take this ignition coil off;
• replace it with the coil from any other cylinder;
• connect both coils back to the correspondent wires;
• turn on the engine and see if the problem is still torturing the unit;
• look up the code and see if it has changed from P0303.

If the code is still the same, the problem is not with the coil. But if it has changed and now shows a misfire in another cylinder, the coil is the culprit of the problem. Buy a new coil and replace the broken one.

3. Compression in the cylinder check

You can't check compression without professional equipment. Also, you may not even know the normal compression for your engine. So, the best option is to go to the dealership or to any trusted repair shop and have your engine inspected by specialists.

In some engines, when you overtighten the spark plug, the problem with the head may happen - it may crack. It's made of aluminum which is pretty soft and can't bear any abnormal pressure. When it cracks, the head will need replacement.

But the loss of compression may also have other reasons, so before you make any conclusions, you will have to inspect the engine. Unfortunately, no DIY recommendations are available here.

What are the symptoms of cylinder 3 misfire?

You need to worry about the health of the engine if after the spark change you encounter such symptoms:

• the engine shakes and shudders significantly;
• idle RPM is not stable and moving up and down all the time;
• it's hard to accelerate, the engine doesn't deliver proper torque;
• the power loss is significant;
• there is a check engine light on the dash of the vehicle;
• the code in the ECU says P0303 meaning Cylinder 3 Misfire.

Also, you will notice problems with gas mileage, possible stalling now and then, and other symptoms. If you experience these symptoms in your vehicle, you may be sure that there is a problem with engine misfire and you should solve this issue as soon as possible.

Can you drive with a misfiring cylinder 3?

Yes, you can still drive a couple of miles and go to the repair shop or to the garage in your home. But bear in mind that when one of the cylinders doesn't work, your engine experiences heavy issues that can affect its longevity and overall condition.

Among the problems that you need to know about are backfire, high temperature in catalytic converter, overheating of the engine, explosions in combustion chambers, harsh work and possible extensive wear, and also oil dilution because of unburnt fuel.

If you keep driving for some days with these issues in your engine, it may just break and require quite complicated and expensive repairs.

How much is it to fix the P0303 error code?

The price of the repair will depend on what exactly happened to your vehicle. In some cases, the problem is easily solved for zero dollars by just tightening up the plug that wasn't tightened properly. Also, if you can locate the issue by yourself and then buy a new plug and install it, the price of the repair will equal the price of the new spark plug for your engine.

But, if you aren't that lucky today, the price of repair of the P0303 or Cylinder 3 Misfire problem can be extremely high. For example, to solve the issue with the cracked head, you will have to pay for a head replacement. This is not going to be cheap for any kind of car.

Anyway, you need to start with locating the issue, and then only you can come up with an idea of how much it can cost you to solve the problem.

Final thoughts

Although the P0303 code after spark replacement doesn't seem that dangerous, you need to act quickly. While in most cases, the problem will be with the spark plug or the ignition coil, sometimes, the issue may be much more expensive to solve. If you can't locate the problem using those simple DIY guides I've mentioned above, better go to the repair shop and let professionals do their job. This way, you can save your vehicle from even more problems.