Thread: 1997 Nissan Pickup Missfire

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the cat is plugged the o2 sensor has to be able to change from rich to lean vary rapidly so the computer can meter the proper fuel mixture this change is called cross counts the older the o2 gets the slower it can respond so you end up with a rich mixture that the cat cant burn the cat is an after burner with the rich mixture there is not enough oxygen left over in the combustion cycle for the cat to get up to temp and burn it off so it coats the catalyst in the converter and it can no longer work and plug's up ad to that this car was driven a long time with the check eng. light on any time that light is on that takes the computer out of the loop and the eng. runs on preset values witch are dead rich and retarded timing that is called limp in mode when that greatly adds to the problem hence the terms open loop (no computer control its running on the prom ) and closed loop when (the computer is controlling the eng.) this is why it is SO important to service your car SOON after that light comes on my best assessment is the cat is dead......ted