Recurring P0420 code on a good running Cavalier

[Moody Blues]

I've got a problem w/ my wife's 97 Cavalier LS, 2.4L. It gives a code P0420 and if you clear it, it takes "about a week" to come back. I cleared it just before we drove from San Diego to Big Bear (about 350 miles round trip, on Interstate 15, high speed and low speed mountain runs, and some city driving) It runs beautiful, but the check engine light came back on just as we got home from the trip. Code P0420 again, cleared right now and waiting for it to come back on.

We did find that the rear O2 sensor had come loose (unscrewed) and retightened it into it's hole. But after a reset, the code came back again.

I ran a voltage test on the rear O2 and it was in specs but jumped around a lot, the Haynes manual said it should be pretty steady. .

Any ideas?

[biggbabysweetz1]

well looking up the code i find this ...
"P0420 Catalyst System Efficiency Below Threshold (Bank 1) " now since im not a rocket scientist and cant speak "code reader" im going to assume that these may be your culprits 1 or more O2 sensors, or you may need to check around your catalytic converter for leaks and such. are the wires going to the o2 sensors frayed or damaged?
have you ever used a leaded gasoline or a lead addative? even using those just once can permanently screw your emissions system up (unless your vehicle says to use leaded of course)

[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.

[maxwedge]

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.

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.

[Moody Blues]

Thanks for the good, well explained advise, though not what I wanted to hear.

When I checked the rear O2 sensor output w/ a voltmeter, it fluctuated between 200 and 700 millivolts. Was kind of jumping around a lot though, not steady like I expected it to be.

That jumpiness and the fact that the sensor had come partially unscrewed make me want to suspect the sensor. But at $80 for a new one, I hate to put it in and then find out it's the cat (or even the front O2 sensor)

And if it is a bad cat, could it damage the new sensor???