| CAUTION | If ECM replacement is instructed in following testing, always ensure ECM harness connector and ground circuit are okay. If either are suspect, repair and repeat testing to confirm ECM malfunction. If ECM is replaced, ECM must be programmed with proper ignition key code for engine immobilizer system. For programming procedures, see COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION. |
Note. Bank No. 1 refers to bank which includes cylinder No. 1. Bank No. 2 refers to bank without cylinder No. 1. Heated oxygen sensor No. 1 refers to sensor closest to engine block.
Circuit Description
Heated oxygen sensor monitors exhaust gas oxygen content and delivers an input signal to ECM. ECM uses input signal to determine fuel injection system operation. Heated oxygen sensors include a heater. DTC is set when heated oxygen sensor No. 1 does not output a rich condition once engine is at normal operating temperature, engine speed of more than 1500 RPM and vehicle speed is 25-62 MPH for at least 2 minutes. Possible causes are
- Heated oxygen sensor No. 1 circuit is open or shorted.
- Vehicle has run out of fuel.
- Fuel system malfunction.
- Defective fuel injector.
- Ignition system malfunction.
- Exhaust system leak.
- Defective heated oxygen sensor(s) No. 1.
- Defective ECM.
Diagnostic Aids
After confirming DTC P0125, use scan tool to access CURRENT DATA to confirm voltage output of heated oxygen sensors (banks No. 1 and 2, sensor No. 1). If voltage displayed on scan tool is always less than 0.1 volt, circuit may be open or shorted. Using scan tool, read freeze frame data. Freeze frame records engine conditions when malfunction is detected.