P0300 Code after Changing Spark Plugs – What's Wrong?

If you get a P0300 code after changing your spark plugs, it means that the engine misfires in different cylinders, so the plugs are faulty or improperly installed. This is especially easy to locate if before changing the spark plugs your car didn't throw this code and everything worked normally without any misfires and other problems.

P0300 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

What's the P0300 code after changing my spark plugs?

So, things were quite good with your car but you decided to change the spark plugs because it was time for this. You bought the new set of plugs, unscrewed the old ones, installed the new ones, connected them, and happily started the engine. But then things went completely wrong. You felt that the engine worked poorly and after some time, you saw the check engine light illuminating on the dash.

The P0300 code showed up when you connected the OBD scanner and things became even more tangled. What's going on with the engine and why it started after you installed the new spark plugs? Let's try to figure this out.

Here are the possible reasons:

1. The spark plugs are faulty

Even though the spark plugs you installed are new, they may still be faulty. You may take them out and go to one of the good repair shops with them to have the plugs checked. The shops use a special piece of equipment to check the spark plugs and they will tell you if the units are OK.

Also, you should check if these spark plugs are compatible with your vehicle. There are several parameters that should be considered.

Also, you may try installing the old spark plugs back. If the problem disappears, you will know for sure that the problem is with the faulty plugs. Then, you may return them to the seller and buy new ones.

2. The spark plugs are installed incorrectly

It's not that uncommon for people without massive experience in car repair to install spark plugs incorrectly. They may overtighten the plugs and leave a tiny space between their working parts or they may tighten it too weakly and cause the wrong position of the spark in the combustion chamber.

The average recommended torque s 10-15 Nm but some engines have other specifications. If you don't have a torque wrench, you better get it unless you perfectly know the needed power to tighten the plugs. Otherwise, the problems can be much more serious than just P0300 and a harshly working engine. The overtightened plug can even harm the engine head.

3. Bad connection with the coil

One more common thing that you may want to check is the coil connection. When changing spark plugs, you will take off the coils. After installing the new plugs, you will put the coils back on and tighten them. It's absolutely clear that when putting the coil onto the plug, you may do something wrong.

For example, a poorly tightened coil may vibrate and give intermittent signals to the plug. Also, it can kill the plug and eventually kill itself very fast. So, the spark plug replacement may not seem so easy-to-accomplish procedure anymore, right?

4. Damaged wiring

When taking off the coils, you may have damaged coil wiring. Even if it happens in one cylinder, the P0300 code may appear. Your ECU isn't always capable of identifying the number of the cylinder that misfires, so it will throw the P0300 code and will make you think about the worse problems than one damaged wire.

Check the wires in the coils and see if some connections are loose. This may help you solve the problem without even taking off the spark plugs. Usually, loose wires are easy to locate and this will only take you a couple of minutes.

5. Other problems

Unfortunately, the problem that triggers the P0300 may not be connected to the changed spark plugs. This is a wild coincidence but it's not impossible. Maybe, you have a problem with clogged injectors, too high or too low fuel pressure, air supply blockage, or some other issue that causes misfiring.

If you can't locate the problem in the new spark plugs, you should inspect the engine systems and find the actual culprit of the P0300 code. In a lot of situations, this will help you save time and money.

Solving the problem with the misfiring engine

So, after you learn what the problem may actually be, you should pay attention to the possible ways to solve the issue. It's not that hard to understand what actually is causing the problem if you take the car in for an inspection. This will cost you about a hundred bucks but will immediately give all the needed answers.

If you decide to inspect the vehicle on your own, here's what you may go with:

• check the connections and wires in your ignition coils - this is the first step;
• make sure all the ground wires are connected firmly and the battery is OK;
• after that, take off the malfunctioning plugs and check them;
• install the old plugs and see if the code goes away after some time;
• buy the new set of plugs - you may want to go with OEM plugs or at least get the product of a good brand;
• if nothing helps, go to the shop and have the car inspected for other related problems.

This is the algorithm that can help you do all the DIY inspections on your own and avoid spending your money on simple things. If you still can't locate the problem this way, you will most likely need to go to professionals.

By the way, the P0300 code will not clear quickly. If you install the old plugs and just start the engine, the code may still be there for some time. So, you may need to drive a car for several hours or just clear the code with your scanner and see if it reappears.

Can you drive with the P0300 code in your car?

Misfiring engine may not sound like a major problem and you can think that you can drive several days till you get the salary or till you have some more time to address the issue. But no, this is an extremely bad idea.

You can drive several miles till you get home or to the shop - that's OK. But only if the engine goes more or less well and doesn't sputter heavily, doesn't knock or stall. If you experience total power loss, it means that the engine misfires heavily, and driving it may end up being an extremely serious issue.

A badly misfiring engine will not burn all the fuel. Part of the unburnt gas will get into the catalytic converter and explode there. It may destroy the converter quite quickly and make you spend thousands for its replacement.

Also, the part of the fuel may go down the engine and dilute your oil. This causes poor lubrication, overheating, and other kinds of problems with the engine. So, driving more than several miles with the P0300 fault code in your vehicle is not a good idea and it may eventually cost you a lot of money.

Proper spark plug replacement - what do you need to know?

If you replace the spark plugs properly, you shouldn't trigger any new codes with this. Here's what you should remember if you decide to do this on your own:

• Make sure your engine is clean - any debris can fall into the spark plug well and get directly into the engine which is extremely bad.
• Look at the new spark plugs and see if they have any plastic covers and temporary shields on them, remove everything.
• Remember about the needed torque when tightening the new plugs.
• Make sure the coils are clean when you put them on the new spark plugs.
• After starting the vehicle with the new plugs, let it idle for a while and see if it works well.

If something is wrong with the vehicle, don't let it idle for long, and don't try to drive it. Use the scanner and read the codes first. If you see the P0300 code, use this article as a guide to find the problem.

Final thoughts

I hope you will never get in the situation when your new plugs will throw some codes and light up the check engine light. I recommend having your spark plugs replaced in the shop as part of the overall maintenance. But if you decide to do it on your own, you should follow the tips given above. Otherwise, the P0300 code is one of the lightest problems you can get after an improper spark change.