Skip the auto shop - Our mechanics make house calls

Q: Misfires on cylinder 3

asked by on

My check engine light started coming on last week. I pulled the code: P303 - Misfire on cylinder 3. The car has a little less than 150,000 miles on it. The spark plugs were replaced near 100,000. The valve cover gasket was leaking near cylinder 4 at the time. That got fixed under warranty. About 20,000 miles later, the same gasket was leaking, so it got replaced under warranty again. Now it's leaking again. I've tried the following to fix the misfires: Swapped the coils between cylinders 2 and 3. I reset the codes, but still got a new P303 reading. I tried swapping the spark plugs, and got the same code. I tried replacing my spark plugs with ones from a different manufacturer. Later, I swapped out my fuel injector, but STILL got the same code. I used grounded plugs with the coils on 2 and 3 last night with a friend, and the sparks look identical and clear. I tried driving 20 miles, and while the plugs were still warm, I did a compression test. (175, 165, 120, 180). Could my cylinder 4 coil be soaked from an oil leak, and would that cause a misfire? I never checked cylinder 4, and I'm currently driving a different car. Do I need to check out the 4th coil and maybe try swapping it out? Or might something else be going on?

A: Hello. Instead of only replacing the coils ...

Hello. Instead of only replacing the coils between cylinders 2 and 3, I think you should replace all of the coils with new coils, if you haven’t done so yet. The results of your compression test show that you have good compression on all of the cylinders, so it’s pretty safe to say that you don’t have an internal engine problems.

Once you change all of the coil packs, it would be a good idea to inspect the wiring going to the coil packs and fuel injectors. It would also be wise to inspect the wiring harness going from the injectors to the Powertrain Control Module (PCM), and wiring harness going from the coil packs to the Powertrain Control Module (PCM).

It is also possible that the Powertrain Control Module (PCM) may be going bad. This could be the answer for why the spark plugs aren’t firing like they are supposed to. Before considering whether or not the Powertrain Control Module (PCM) is the problem, your first step should be to have your coil packs replaced by a certified technician.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%

Get a quote

What others are asking

Q: Car won't start after sitting for a year.

If a vehicle has not been started for a year, then you may have a failed fuel pump and the fuel inside the tank may be degraded and can cause problems like a clogged fuel injector. The recommendation I would...

Q: Engine smells hot at times.

Hello. If the check engine light is coming on at times with no other symptoms then the best thing to do would be to have the computer scanned for codes as that can reveal what the engine is seeing. If...

Q: How do you operate the Turn Signals?

Your Turn Signals provide important directional information for drivers behind you as well as those approaching you. They tell other drivers that you’re about to turn, so they can expect to slow down and will begin backing off (it also...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...