| CAUTION | If ECM replacement is instructed in following tests, always ensure ECM connectors and ground circuit are okay. If either are suspect, repair and repeat testing to confirm ECM malfunction. |
Note. Heated oxygen sensor No. 1 refers to oxygen sensor closest to engine block. Heated oxygen sensor No. 2 refers to oxygen sensor farthest from engine block.
Note. For component location, see appropriate illustration in THEORY & OPERATION article.
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. DTC is set when heated oxygen sensor No. 1 does not output a rich condition once engine is at normal operating temperature, engine speed is more than 1500 RPM and vehicle speed is 25-62 MPH for at least 2 minutes. Possible causes are
- Fuel system malfunction.
- Air induction system malfunction.
- Defective fuel injector.
- Ignition system malfunction.
- Exhaust system leak.
- Heated oxygen sensor No. 1 circuit is open or shorted.
- Defective heated oxygen sensor No. 1.
- Vehicle has run out of fuel.
- Defective ECM.
Diagnostic Aids
After confirming DTC P0125, use scan tool to access CURRENT DATA to confirm voltage output of heated oxygen sensor No. 1. If voltage displayed on scan tool is always less than .1 volt, circuit may be open or shorted. Using scan tool, read freeze frame data. Freeze frame records engine conditions when malfunction is detected.