| 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 - IMPORT article in GENERAL INFORMATION, or appropriate ENGINE IMMOBILIZER SYSTEMS article in ACCESSORIES & EQUIPMENT. |
Note. Bank 1 is also known as right bank (RH). A/F sensor refers to sensors closest to engine block.
Note. For component location, see appropriate illustration in THEORY & OPERATION article.
Circuit Description
Air/Fuel (A/F) sensor monitors exhaust gas oxygen content and delivers an input signal to ECM. ECM uses input signal to control air/fuel ratio. A/F sensor includes a heater. DTC is set when A/F sensor output does not change with engine speed at 1500 RPM or more, vehicle speed 25-62 MPH and throttle valve not fully closed for at least 1.5 minutes. Possible causes are
- A/F sensor circuit is open or shorted.
- Defective A/F sensor (bank No. 1 or 2, sensor No. 1).
- Air induction system malfunction.
- Fuel system malfunction.
- Defective injector.
- Running vehicle out of fuel.
- Exhaust system leak.
- PVC hose connection.
- Defective ECM.
Diagnostic Aids
After confirming DTC P0125, use scan tool to access CURRENT DATA to confirm voltage output of A/F sensors (bank No. 1 and 2 sensor No. 1). ECM controls voltage of AFR+, AFL+, AFR- and AFL- terminals at ECM to a fixed voltage (3.3 volts at AFR+ and AFL+ terminals; 3.0 volts at AFR- and AFL- terminals). It is impossible to confirm A/F sensor output voltage without using a scan tool. A/F sensor output voltage on OBD II scan tools is displayed at one fifth the voltage of that displayed on Toyota hand-held tester. Using scan tool, read FREEZE FRAME data. Freeze frame data records engine conditions when malfunction is detected.
During fuel enrichment, output voltage of A/F sensors may be less than 0.56 volt (2.8 volts on Toyota hand-held tester). During fuel cut, output voltage of A/F sensors may be greater than 0.76 volt (3.8 volts on Toyota hand-held tester). If output voltage of A/F sensor remains at 0.66 volt (3.3 volts on Toyota hand-held tester) during all conditions, A/F sensor circuit may be open. If output voltage of A/F sensor remains at 0.76 volt (3.8 volts on Toyota hand-held tester) or more during all conditions, A/F sensor circuit may be shorted. If output voltage of A/F sensor remains at 0.56 volt (2.8 volts on Toyota hand-held tester) or less during all conditions, A/F sensor circuit may be shorted.