Circuit Description

Oxygen sensor monitors exhaust gas oxygen content and delivers an input signal to ECM. ECM uses this input signal to determine fuel injection system operation. Sensors include heater. DTC P0125 is set when sensor does not output a rich condition once engine is at normal operating temperature, engine speed is 1400 RPM or more, vehicle speed is at 25-62 MPH, throttle valve is not fully closed, and at least 180 seconds have elapsed since engine has been started, for at least 1.5 minutes. Possible causes are:

• Open or short in oxygen sensor circuit.
• Faulty heated oxygen sensor (Bank No. 1 or 2, Sensor No. 1).
• Faulty air induction system.
• Incorrect fuel system pressure.
• Fuel injectors.
• Exhaust system leak.
• Faulty ECM.