P0138 — O2 Sensor Circuit High Voltage Bank 1 Sensor 2

Diagnostic algorithm

1. Read and record all codes and freeze frame data. Clear codes and run drive cycle to see if P0138 returns.
2. Perform visual inspection of bank1 sensor2, connector, and harness for physical damage, heat, or contamination.
3. With a scan tool, monitor bank1 sensor1 and sensor2 voltages and fuel trims at idle and while varying engine load. Note if sensor2 is consistently high (>0.9V) or mirrors sensor1.
4. Check for a short to battery: with ignition ON (engine off) backprobe signal wire — if ~12V present, inspect wiring for pinched/shorted sections or aftermarket taps.
5. Check continuity/resistance from sensor signal pin to PCM signal pin; verify good ground(s) associated with sensor circuit.
6. Inspect and test heater circuit (if equipped): verify supply voltage and heater resistance per vehicle spec. A failed heater alone normally won’t force a high signal but indicates sensor age/damage.
7. Evaluate fuel system: measure fuel pressure and inspect injectors for leaks; review long‑ and short‑term fuel trims for signs of a rich condition.
8. If wiring and fuel system check OK, swap or replace the bank1 sensor2 with a known good unit (or temporarily swap with the same type on another bank if vehicle design allows) and retest.
9. After repair or replacement, clear codes and perform drive cycle. If code returns and wiring/supply verified, consider PCM fault and consult manufacturer service information.