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knisalad 4/18/2015

2003 Pontiac Bonneville SLE 6 Cyl 3.80L


p300 p420 engine shaking/loud at idle and in gear

I've had this problem for over a year now. sometimes car idle smoothly, others it chugs hard, almost stalls itself out/ battery fluctuates. When accelerating easy it runs normal. when accelerating heavy there is a loud chirping/ squeeking noise, then the engine shakes violently, and has very little power. Replaces MAF sensor/ air filter one year ago. Spark plugs/ fuel filter 2 years ago.

1 Answer


Jimm 4/18/2015

P0300 = Random Cylinder Misfire

For the misfires - It may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

Possible Solutions
If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

P0420 = Catalyst system efficiency below threshold (bank 1)

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
A damaged or failed oxygen sensor (HO2S)
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Leaking fuel injector or high fuel pressure
Cylinder misfire
Oil contamination


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