Note. This DTC may be caused by the vehicle running out of fuel.
| CAUTION | If ECM replacement is instructed in following tests, always ensure ECM connectors and ground circuits are okay. If either are defective, repair and repeat testing to confirm ECM malfunction. |
Note. Heated oxygen sensor No. 1 is located on exhaust pipe in front of catalytic converter and may be referred to as bank No. 1 sensor No. 1. Heated oxygen sensor No. 2 is located on exhaust pipe in behind catalytic converter and may be referred to as bank No. 1 sensor No. 2.
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 Toyota hand-held tester or scan tool to access CURRENT DATA to confirm voltage output of heated oxygen sensor No. 1. If displayed output voltage is always less than .1 volt, circuit may be open or shorted. Using hand-held tester or scan tool, read FREEZE FRAME data. Freeze frame data records engine conditions when malfunction is detected.