Note. This DTC may be caused by the vehicle running out of fuel.
| CAUTION | If ECM replacement is instructed in following testing, always ensure ECM harness connector and ground circuits are okay. If either are defective, 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. Heated A/F sensors are located on each exhaust manifold. Heated A/F sensor on rear (firewall side) exhaust manifold may also be referred to as bank No. 1 sensor No. 1. Heated A/F sensor on front (radiator side) exhaust manifold may also be referred to as bank No. 2 sensor No. 1. A/F sensors may also be referred to as A/F ratio sensors.
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 140 seconds. 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.
- Improper fuel pressure.
- Defective fuel injector.
- Exhaust system leak.
- 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 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+ terminal; 3.0 volts at AFR- and AFL- terminal). It is impossible to confirm A/F sensor output voltage without using Toyota hand-held tester or scan tool. A/F sensor output voltage on scan tool is displayed at one fifth the voltage of that displayed on a Toyota hand-held tester. Using hand-held tester or scan tool, read FREEZE FRAME data. Freeze frame data records engine conditions when malfunction is detected.