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Originally Posted by Moody Blues
The only additive ever put through the car is Techron, which I've had very good luck with. The exhaust is tight, and the wiring looks good.
Based on what I've read/learned, it is either a malfunctioning O2 sensor, of a dying catalitic converter.
Was wondering if anyone had a clue which one it is based on prior experience, etc.
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This can be a tough diagnosis, the post cat sensor is supposed to hold fairly steady at about 650 mv and and cycles at approx 1/2 the rate of the # 1 sensor, this can only be checked on a graphing scanner or scope. Excess oxygen in the exhaust causes the sensor to set a code as the pcm assumes the cat can no longer utilize the oxygen to do its job. In lieu of the proper equipment here take a chance with a new sensor keeping in mind a skewed pre cat sensor can fool the post cat sensor to set the code. Googling p0420 will find some more definitve info, good luck.