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View Full Version : O2 sensor, Cat converter, rough idle



rccolacc
05-15-2005, 05:42 PM
I bought a '97 Grand a few months ago. It had 120K miles on it and had a rough idle. I did a tune up (plugs, wires, and cap and rotor), and it didn't do much to it. I replaced the idle air control motor right after I bought it too, thinking that was the problem. Recently I got a new cat converter and the upstream O2 sensor, and it didn't help either. It keeps throwing a check engine code 21 (something to do with the O2 sensor) and the exhaust smells really rich. Do I need to replace the downstream O2 sensor too? With gas being so expensive these days, I'd like to fix this.
Thanks.
-RC

OverkillZJ
05-15-2005, 05:44 PM
I think downstream usually has more of an effect than up stream. at 120k, might as well replace it anyway, I bet it fixes your issue too.

Edit: I lied^^^

TrojanMan
05-15-2005, 10:37 PM
Pre-cat sensor adjusts your A/F ratio, post-cat sensor only tells you if the cat is working or not. Pre-cat sensor is very important, post cat is not important at all.

dasVettemeister
05-28-2005, 01:01 AM
The post-cat O2 has one purpose: It will set a code per OBDII regulations to tell you that emission parameters have been exceeded. Otherwise it has no purpose. That is why you can get away with not running a post-cat O2 and just plugging in an emulator. Pre-cat O2 adjusts A/F ratio, but only at partial throttle settings. At full throttle settings, A/F ratio is controlled by fuel curves existing in the computer programming referenced by IAT sender which helps determine engine load. There, no more fairy-tales.

OverkillZJ
05-28-2005, 10:01 AM
The post-cat O2 has one purpose: It will set a code per OBDII regulations to tell you that emission parameters have been exceeded. Otherwise it has no purpose. That is why you can get away with not running a post-cat O2 and just plugging in an emulator. Pre-cat O2 adjusts A/F ratio, but only at partial throttle settings. At full throttle settings, A/F ratio is controlled by fuel curves existing in the computer programming referenced by IAT sender which helps determine engine load. There, no more fairy-tales.

Yup, you're right, I take it back. I mixed them up. I had a derrrrr moment.

dasVettemeister
05-28-2005, 12:17 PM
The post-cat O2 has one purpose: It will set a code per OBDII regulations to tell you that emission parameters have been exceeded. Otherwise it has no purpose. That is why you can get away with not running a post-cat O2 and just plugging in an emulator. Pre-cat O2 adjusts A/F ratio, but only at partial throttle settings. At full throttle settings, A/F ratio is controlled by fuel curves existing in the computer programming referenced by IAT sender which helps determine engine load. There, no more fairy-tales.

Yup, you're right, I take it back. I mixed them up. I had a derrrrr moment.

I have those moments VERY frequently. :smt003 I just happened to have the brain in gear for a few rare moments last night!