Hey everyone... My parents have a '97 STS that has been giving them fits... Here'* the info grabbed from it through Autotap... I wanted to get your opinion before getting gun-ho at replacing parts. For what it'* worth, my dad likes to take the shotgun approach and so far it hasn't fixed the problem...

DTC'* Set in '97 Cadillac STS
H P0135 HO2S Heater Circuit Bank 1 Sensor 1
H P0141 HO2S Heater Circuit Bank 1 Sensor 2
H P0147 HO2S Heater Circuit Bank 1 Sensor 3
H P0404 EGR System
H P0603 PCM Memory Reset
H P0135 HO2S Heater Circuit Bank 1 Sensor 1
H P0141 HO2S Heater Circuit Bank 1 Sensor 2
H P0147 HO2S Heater Circuit Bank 1 Sensor 3
H P0404 EGR System
P P0135 HO2S Heater Circuit Bank 1 Sensor 1
P P0140 HO2S Circuit Insufficient Activity Bank 1 Sensor 2
P P0141 HO2S heater Circuit Bank 1 Sensor 2
P P0147 HO2S Heater Circuit Bank 1 Sensor 3
P P0155 HO2S Heater Circuit Bank 2 Sensor 1
P P0404 EGR System

All of the below observations were done with the car idling:
The HO2S curves look similar to my bville, with Bank 1 sensors 1&2 fluctuating between ~300-800mV (with the car cold), however, Bank 1 Sensor 3 is at an almost steady 900mV and Bank 2 Sensor 1 fluctuates between 400-600mV.

Once I warmed the car up, Bank 1 Sensor 1&2 were fluctuating between 100-900mV, Bank 1 Sensor 3 was a steady 900mV, and Bank 2 Sensor 1 was fluctuating between ~200-800mV. While going down the road, each sensor seemed to follow each other in terms of how the curves looked. When one would go high, the rest would follow...

Long Term Fuel Trims Bank 1 is a fairly steady -12% and Bank 2 seems to stay right at 3% (again with the car cold). Once I warmed it up, LTFT Bank 1 was at a steady -18.8% at idle and bank 2 seemed to bounce around a bit, but nothing over -3% and nothing higher than 0%.
Short Term Fuel Trims seem to bounce around a lot but stay between 0% and -10% mostly around -3% to -6% (cold)... Warmed up STFT seem to both stay between 2% and -5%.

Now when I looked at the short term fuel trims from the various O2 sensors, Bank 1 Sensors 2&3 showed 99.2% constant. Does this mean that these 2 O2 sensors have crapped out?

The car exibits no real driveability concerns, but the Check engine light never seems to go away. We have replaced the 2 O2 Sensors that are right on the manifolds which I believe are Bank 1 Sensor 1 and Bank 2 Sensor 1. Plugs and wires were just done about 4 months ago... These O2 sensor issues are driving me nuts, but the only thing my parents complain about is that it only gets around 17mpg when they used to get 24mpg...

I think atleast the STFT from O2 Sensors Bank 1 Sensor 2&3 point towards bad sensors, but I'd like your opinion. I don't like all those DTC'*...

Click here to download a zip file containing my Autotap config file, Autotap log file, and an excel file created from the Autotap log file.

TIA!

I think its a wiring issue?
I didnt know this about the O2 sensor? and it would be easy to have broken insulation given the heat of the engine bay and age of the wires?

Thanks for the reply. It turns out that the EGR valve was causing everything else to go haywire. $116.23 + $23.25 shipping for a new EGR valve from GMPartsDirect.com and all is well in Cadillac land for my parents.