Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Check engine light won't go away

2008 Chevrolet Impala LS 3.5 V6 flex fuel
Check engine code: P0304

Please help my horror story. I purchased my Impala 2 years ago, love my car. However, there has been a shadow in the form of a check engine light that I have always had. I purchased the car, the check engine light came on: P0304, multiple maintenance trips and multiple fixes over these two years.

  • changed spark plugs
  • Replaced catalytic converter
  • Switched to premium octane gas
  • Purchased new gas cap
  • Tried two different fuel injectors

No matter the attempt for resolution, It comes back with P0304. The car otherwise runs fine, but I’m tired of having the check engine light on, especially since it disables the automatic start feature on my key remote. Any ideas on where to go next?

The CEL system is very poorly designed and will not identify a problem but merely hints that there is a problem with your engine. You need to take your Impala to a mechanic who is qualified and experienced enough to find the root of the problem. It’s not easy but you could spend months and a boatload of money by using the “shotgun” method of mechanical troubleshooting. P0304 is a hard problem to pin down. Here are some common causes of the fault:

Worn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable)
Incorrect ignition timing
Vacuum leak(s)
Low or weak fuel pressure
Improperly functioning EGR system
Defective Mass Air Flow Sensor
Defective Crankshaft and/or Camshaft Sensor
Defective Throttle Position Sensor
Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems

That code indicates a misfire in cylinder #4, so whoever told you to replace the catalytic converter was leading you down an expensive–and incorrect–path. Since changing the spark plugs didn’t resolve the problem, I would suggest that you consider the coil for the #4 cylinder as the probable cause of the problem.

3.5 uses coil pack. not too pricey. $50 range. i would have tried that right after the plugs.

Coil pack and a new set of wires would be my guess to clearing this PO304 code.

The converter, and gas cap would have no effect on the misfire code.
You could have eliminated the injector too simply by swapping the #4 injector with any other one and see if the misfire moves to that cylinder.


If PCM is flagging misfires yet the car runs fine, there may be an update for your PCM to address this.

Here’s the list I found, sounds like coil is next to try:
•Faulty spark plug 4 [done]
•Clogged or faulty fuel injector 4 [done]
•Faulty ignition coil 4
•Fuel injector 4 harness is open or shorted
•Fuel injector 4 circuit poor electrical connection
•Ignition coil 4 harness is open or shorted
•Ignition coil 4 circuit poor electrical connection
•Insufficient cylinder 4 compression
•Incorrect fuel pressure
•Intake air leak

Ah ha, bad harness connection which could be there from new. Wouldn’t be the first time.

A misfire is usually caused by one or more of these

  • spark problem
  • fuel injector problem
  • compression problem

So your shop just has to go through that list one by one. Spark and fuel injector problems are usually diagnosed by swapping parts from number 4 to another one that is working correctly. If the problem moves to the other cylinder, then you know that part is the problem. Parts to swap include the spark plugs the coil packs and the injectors. Testing the compression in all four cylinders is a simple test that any shop can do for you.

If nothing from the above is found to be the cause, and there’s no obvious symptom other than the check engine light it could be a problem with the the computer sensing the crankshaft position. Computer problem, or crankshaft position sensor, or the thing it senses as the engine rotates.