"Do normal O2 sensors have a variable signal that they send to the PCM? Like if the cats were doing one thing the O2 sensor would know that and the PCM would add more or less fuel?"
The upper O2 sensors (as well as other sensors but not lower O2 sensors *) will send signals to the PCM and have the PCM adjust the air/fuel mixture.
"The reason I ask is that my fuel mileage is crap and I wanted to replace my o2 sensor(s)."
If you have 100k miles or so on your upper O2 sensors, it might help.
"If I use MIL eliminators instead will they act like brand me o2 sensors and help fuel mileage?"
Mileliminators are for the lower O2 sensors only. In a previous thread, it was mentioned by two CEGers that the P0420 and P0430 codes do not affect fuel mileage. I do not agree * but do not have any technical documents to back up other than my personal experience with my fuel mileage being down when the CEL for P0420 was lighted (maybe there were also other issues or codes that I did not know about).
Last edited by Tony2005; 04/19/06 08:54 PM.
