OBDII Reading (Part 2)

Ok, here is what is going on since my last post. My check engine light came on again so I had autozone give me the reading once again to give you guys the exact reading. The results: P0420 (Catalyst Efficiency Below Threshold, Bank 1). The background: I had a new Catalytic converter put on a year ago, shortly after that, my light came on, I took it back and had them put another cat on and again the light came on eventually. The light comes on randomly about every 2-3 months when driving highway speeds usually. I usually reset it and keep on trucking. Autozone told me to replace the O2 sensor however, this thing has a half dozen of those and they are 50 bucks a piece. I dont really want to replace all of them if I dont have to, should I take this to GM and pay them the 80 bucks to give me a more thorough reading or has anyone else encountered such a problem and had a fix for it. I did replace the first O2 sensor behind the cat but that didnt work either. Any suggestions would be greatly appreciated. Thanks. :banghead:

Any help out there on this one???

somebody has to know something about this....

somebody has to know something about this....

I am experiencing the same problems. Took it to the dealer and they replaced 2 - o2 sensors and did some other stuff (see my post under 1997 service engine soon light, need help) and I paid $1,000. Took it on the road and went about 250 miles and the light came back on with a loss of power going uphill. Took it back to the dealer, they said it now needed a tune-up, had that and took it on the road and the same thing happened. I'm tired of paying money for people guessing at what's wrong.

SOmeone did reply and tell me to replace the gas cap (which is on order from auto-zone). This only seems to happen when I'm on the highway and at intervals of about 250-300 miles. I'll watch the board to see if you get a response. Thought that's what this forum was all about, but guess I was wrong. Good luck....A. Felix

From what I can gather from your other posts this is a 96 Blazer with a 4.3L. Did you have a GM catalytic convertor installed? On the oxygen sensors since this vehicle should only have one catalytic, you should be able to swap positions on the two sensors closest to the catalytic, they should have the same plug on them, the two closest to the engine have a different plug. The code you are getting usually means that the PCM is seeing oxygen sensor values changing rapidly and going over 450mv to the post cat O2 sensor-Bank 1 Sensor 3 on the 4.3 and 5.0 engines, the 5.7 and larger have two cats with two post cat O2 sensors- Bank 1 Sensor 2 and Bank 2 Sensor 2

On my 96 4.3 i have an o2 after the cat, might be the one your looking for.

Ok, your correct, this is a 96, 4.3 v6. I did replace the 1st O2 sensor after the cat the first time someone told me that error code but it did not get rid of the problem, occassionally, the light does return. The cat is not a GM cat, I had some muffler shop put it on. I thought it may be the problem but I did have them replace the cat nine months after they installed it b/c I told them it was defective. They did and yet the problem still occurs. I think I have ruled out the cat and the O2 sensor directly after the cat, that does leave 5 or more O2 sensors to be checked. I think I would like to check the O2 sensors myself to rule those out, does anyone know how to test if a sensor is bad. Thanks for the posts so far.

Have you reset the PCM to clear the code and see if it still comes back?

Ok, you lost me on that one. What does it mean to reset the PCM? All I have done is disconnect the 2 fuses that relate to the guages and it resets the light. Can anyone explain what "reset the PCM" means?

Disconnect the negative on the battery for 1/2 an hour or use a code reader to reset it...

Ok, here is what is going on since my last post. My check engine light came on again so I had autozone give me the reading once again to give you guys the exact reading. The results: P0420 (Catalyst Efficiency Below Threshold, Bank 1). The background: I had a new Catalytic converter put on a year ago, shortly after that, my light came on, I took it back and had them put another cat on and again the light came on eventually. The light comes on randomly about every 2-3 months when driving highway speeds usually. I usually reset it and keep on trucking. Autozone told me to replace the O2 sensor however, this thing has a half dozen of those and they are 50 bucks a piece. I dont really want to replace all of them if I dont have to, should I take this to GM and pay them the 80 bucks to give me a more thorough reading or has anyone else encountered such a problem and had a fix for it. I did replace the first O2 sensor behind the cat but that didnt work either. Any suggestions would be greatly appreciated. Thanks. :banghead:


there are book out there that have test in them ..i have a hanes manual..this will give you the location for the O2 sensor for you make and model ...you will need a multimeter to do the test..make sure you follow the step for the test...there one trest for the O2 side of the sensor with set reading most times it is in ohm's ,,,then there's test for the engine side of the sensor...just follow the book if yiu can find it..best of luck e mail me if you have any problems..

Dude, this thread is 8 months old.