so heres my issue. have a 98. recently had an issue with the downstream O2 sensor circuit.
the wires for the downstream O2 were rubbing on my driveline which caused a short between the 12v heater circuit for the O2 sensor to the 5v ref. signal grounding to the PCM. I repaired the circuit and replaced the downstream O2.
Now the short term fuel trim skews lean at idle and at light throttle with both O2 sensors connected. the upstream O2 sensor cross counts are good but the voltage seems off, the downstream O2 voltage is within spec.
Is it possible that the 12v from the heater circuit back fed into the sensor ground frying the PCM? because if I unplug the upstream O2 the S.T. fuel trim stays constant and doesn't lean under throttle.
HELP!!!