Confusing Codes

  • 1 POST
  • 4.7L
  • V8
  • 2WD
  • 170,000 MILES

Running very rough. Check engine light comes on, but does not stay.
Checked codes and got P0300 and P0308 (misfires) and O2 sensor B2S1 slow response.
Change all spark plugs and coil packs and both O2 sensors on right side of engine.

Same problem.

I am lost. Is it the ECU? One shop told me it could be head gasket leak. But they were not sure.

Jim Stinson
Sulphur, LA

Do you
have the same problem?
Thursday, December 29th, 2016 AT 8:15 AM

1 Reply

  • 29,781 POSTS

Oxygen sensors do not cause misfires. A misfire is a result of air, fuel, spark, compression, or timing. If you are getting the same fault codes after replacing the oxygen sensors, it suggests they were just reporting the condition, not causing it. Timing will not cause a problem on just one cylinder.

A spark-related misfire will send unburned fuel and air into the exhaust system. You will smell the fuel at the tailpipe, but the oxygen sensor only detects oxygen, so it will report a constant "lean" condition. Normal operation is it switches between "rich" and "lean" about two times per second. Staying lean too long will trigger a lean code or a slow response code. You can also get a lean condition from low fuel pressure, a plugged injector, or a vacuum leak.

You might want to switch two injectors, erase the fault codes, then see if a misfire code sets for a different cylinder. Chrysler has very little trouble with their injectors, but you do not want to overlook this possibility. This also might be a good time to do a compression test. If you find the suspect cylinder is low, look for a hole in the piston, (based on your description of rough running), or a damaged lobe on the camshaft.

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Thursday, December 29th, 2016 AT 12:51 PM

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