Edit: Solved. Ground wire on the ECU harness was severed. I soldered the black cable, bundled with the rear O2 signal wire (Pin 54), to the Sensor Ground wire (Pin 92). Voltage readings and readiness monitor completed.



Rear O2 Harness below passenger seat



O2 Sensor Activity: Red is front and green is rear

Troubleshooting a low to no voltage situation with the downstream/rear O2 sensor. On my obd scanner the rear O2 barely registers 0.2volts (Green line in pic). Exhaust leaks fixed. Current setup: 3” DP > OEM 8 Cat. > 3” cat-back. I just purchased a new Denso O2 sensor.

My initial suspicion leads me to a short on the rear O2 sensor ground which is pin 92 on the C-115 ECU harness. But this ground is shared with coolant temp, tps sensor, and these both sensors are working fine.

Should I be chasing the sky blue/ red & yellow signal wires? This CEL is the last thing I need to fix before the smog test. Any help is appreciated!

Ecu Pinout source:
https://evoecu.logic.net/wiki/USDM_VIII_ECU_Pinout)Dow