Does anyone have information on what an acceptable output voltage range is from the O2 sensors? Also, what should the ratio between the pre-cat/post-cat O2 sensors be (any ideas)??

I've got my car hooked up to read live sensor data and noticed my upper (pre-cat) O2 sensor varied between 0.01-0.4 volts. The O2 sensor after the catalytic converter was more constant - in the 0.4-0.9 volt range.

I think the upper O2 sensor is intermittant but I don't have info on what is an acceptable output range. Any help would be appreciated.

FYI this is a 1998 Mystique Zetec with ~112,000 miles (probably the original 02 sensor too). The car also generated a CEL P1131 code recently (Lean Condition).

I think should be .1 - 1.1 vdc. and should operate between .4 - .6 vdc. If you are getting low voltage it will give you a lean code. You can check autozone.com repair section to verify what I've said.

The upstream sensor should be switching rapidly full range between .1 volts and .9 volts. .1 is lean and .9 is rich. The switching would be from the pulsation from the opening and closing of the exhaust valves. If it is staying between .1 and .4 it is reading a lean condition or it is malfunctioning. Try driving the system rich to see what happens to the readings. You can drive it rich by spraying gas into the intake (very dangerous) or by using an unlit propane torch.

The downstream sensor should not switch nearly as rapidly. That is how the computer tells if the cat is functioning well or not. When the downstream sensor too closely mirrors the upstream sensor, the computer says "tilt" and turns on the check engine light and sets a code. I don't know what the ratio paramaters are.

I'd just replace the upstream sensor. The P1131 is a pretty reliable indicator that the sensor is bad.

Steve