Quote:
Originally Posted by andrewwynn
O2 can't range to negative values. The precat if reading negative values something is wrong. Looks like a case of problem sensor.
Also: post cat should gravitate to 0.45V so the 0.75 is very high but that can vary a lot.
What I would do: reset all the sensors by unplug them all, start the car run until you get multiple errors (probably missing O2 heater error) then plug them back in and clear the codes.
See if the numbers will come back into normal range (0.1 to 0.9v).
If you get values outside of normal you probably need to replace the sensor (use OE or OEM don't cheap out: I need to replace my knock off sensors)
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Actually, for Short term fuel trim or long term fuel trim the value can be negative or positive. Negative STFT will mean that the DME is taking away fuel and positive STFT will mean the DME is adding fuel to the mixture.
The post cat 02 voltage should be at a constant value whereas the pre cat 02 sensor the voltage will vary.
Here's a video on 02 sensors as it relates to voltage
https://www.youtube.com/watch?v=zyh_4SktmFM