Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1995 Subaru Legacy BaseFACTORY OEM SERVICE PROCEDURE
DTC P0135 (FO2H) - Front Oxygen Sensor (O2S) Heater Circuit Malfunction
- Turn ignition to OFF. After Connecting SSM or OBD-II scan tool, turn ignition to ON. Turn SSM or OBD-II scan tool to ON. Start Engine. On SSM, select function mode F29 to read front O2S heater current indication. If reading is 0.2 amp or more, repair O2S and/or ECM connector. If current is less than 0.2 amp, go to next step.
- Start and idle engine. Measure voltage between ECM connector B84, terminal No. 44 and ground. If voltage is 1.0 volt or less, go to next step. If voltage is 1.0 volt or less while shaking harness and connector of ECM, repair contact in ECM connector. If shaking does not change reading, go to next step. Disconnect connector from front O2S. Measure voltage between ECM connector B84, terminal 44 and ground. If voltage is 1.0 volt or less, replace ECM. If voltage is more than 1.0 volt, repair short circuit in harness between ECM and front O2S connector. After repair of short circuit, replace ECM.
- Turn ignition to OFF. Disconnect front O2S connector. Turn ignition to ON. Measure voltage between O2S connector B18, terminal No. 2 and ground. If voltage is 10 volts or more, go to step 4). If voltage is less than 10 volts, Repair power supply line between main relay and front O2S.
- Turn ignition to OFF. Measure resistance between front O2S connector B18, terminals No. 1 and 2. If resistance is more than 30 ohms, replace O2S. If resistance is 30 ohms or less, repair open in harness between front O2S connector and ECM connector, poor contact in front O2S connector or poor contact in ECM connector.
Factory Service Tools
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.