1999 Saturn SL, 1.9 L engine, double overhead cam.
Gas mileage is down. Code reader says code P0134, O2, Bank 1 Sensor 1. Replaced O2 sensor on the exaust manifold. Still the same. Checked new O2 sensor with volt meter. It reades what Chiltons says it should. Checked voltage on the wire from the harness going to the sensor, with the sensor unhooked, and it reads what the book says it should. I am still receiving the O2 code. I can't see any wires loose anywhere, nor any vacuum lines. I do have a good bit of oil on the wireing behind the motor, probably coming from the cam gasket. Is there something I'm missing. I'm at a loss.
P0134 = HO2S Circuit Insufficient Activity Bank 1 Sensor 1
Sounds to me like PCM is saying that it isn't seeing enough variation in the O2 sensor reading from one of them. Not sure which bank is which 02 sensor, but you may want to check the 02 sensor after the cat as well.
ODBII Scan tool with real time readout will be able to show you raw readings for both 02 sensor signals (as PCM sees them) -- this should hopefully shed some light on your dilemna.
Also -- have you ever serviced/cleaned your EGR valve ?
Might a malfunctioning EGR (sticking due to carbon buildup) maybe throw 02 sensor codes b/c it upsets the mix of the exhaust gases that the 02 sensor sees, and the pcm doesn't understand this?? A stretch, but maybe worth cleaning EGR (notorious for sticking problems on the S-cars due to buildup from all the oil burning).
I haven't tried cleaning the EGR, but I certainly will. Since I posted, I unhooked the O2 sensor downstream of the cat. I got a different code, something to do with preheater circuit. Hooked it back up and that code went away, so it's seems to be the O2 sensor on the exaust manifold that's sending Bank 1 Sensor 1, O2 sensor not responding.
I'll try the EGR. I hope I can find what's wrong with this thing. It's dropped by mileage down by a constant five to six miles per gallon. I drive a round trip of 80 miles, twice a day, US highway 10 miles, then about 28 miles of interstate, then a few blocks of city traffic, then back home. Then another round trip 12 hours later. Fill it to the top, check my mileage with a calculator every time. Mileage has suddenly dropped from around 39 and 40 to 33 or 34. I usually get 4 round trips with about a quarter tank left. Now I get three trips with a quarter left. I no not accelerate hard, drive a constant 65 on the interstate, and 55 on the state road, trying to stretch the gas. This bugs me, and apparently I'm not knowledgeable enough to find it. So again, thanks for the help. I'll try both things you said.
What codes come up when you do clear the codes and then pull the O2 after the cat? Is it both or just the O2 sensor heater code? And which heater code is it (Pxxxx)?
With the rear O2 disconnected: clear the codes and disconnect the front O2 sensor and tell me which codes set. I would expect you to get a heater code on the front O2 (if the front sensor is indeed also heated) as well if we are on the right track as far as bank 1 / bank 2 (actually I didn't think any S cars used heated 02 sensors---the older ones didn't). Does your Chilton book indicate that both are heated 02 sensors?
If the front sensor is indeed also heated, but the heater code doesn't set when disconnected, it may be that the issue is in the heating circuit for the sensor. Seems plausible that an O2 sensor which is not heated to its optimal temp would have a hard time responding properly if it is heated just enough to be within its operating range but way at one end of the range.
Again, real time diag data via from PCM via ODBII should help quite a bit here.