2002 SE, F23A1 is occasionally experiencing severe RPM drops when coming off the throttle, has now thrown a CEL with a P0130 (low voltage to pre-cat O2 sensor, Bank 1). Double digit negative STFT and LTFT values present, but STFT values approach 0 before the O2 sensor signal gets dropped and both it and STFT goes to 0 on live data.

A few points:

1. Only ever occurs after letting off the throttle, like when stopping for a stop sign, revs drop beyond the 700-1000 normal mark for idle, motor almost stalls and then climbs RPM a bit, but still low idle. It's like the RPMs cruise right past where they should stop and even out for idle.
2. Only ever happens when the engine is warm. Has never occurred after a cold start. Appears to be worse the warmer the ambient temps and motor temps are, but that could just be my perception.
3. After clearing the code and driving around for a bit, live data shows both STFT and LTFT in the negative double digits (e.g. -17 to -10), and the primary O2 sensor is oscillating between 0.015 and 0.78 v. No drivability issues or codes. As the STFT gets into the single digits and closer to 0, it will occasionally make it to 0 and then the problem starts, sometimes it starts dropping RPMs after coming off the gas first, or sometimes it will throw a CEL for the P0130 code and then the problem starts. Once the problem starts and the CEL is thrown, the primary O2 sensor reading immediately goes to 0 as does the STFT, and stays that way until the code is cleared. Secondary O2 sensor readings continue unaffected and values are pretty close to what they would be before the CEL.
4. I checked the wiring as well as a quick check for any gross vacuum leaks, didn't find anything.

Question: is this a sure sign of needing to replace the primary O2 sensor, or could there be something else going on? If the sensor is on it's way out, would it's lack of signal result in the code and the ECU shutting it down in some kind of limp mode, or would another problem (IACV, etc) cause the mixture to be out of whack and cause the ECU to cut the O2 sensor signal? What could be causing the lower then desired STFT values?
Thanks in advance!

 

Bump - anyone?