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.
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HOMESERVICE MANUALSSUBARU1998FORESTER L, STANDARDREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODESDIAGNOSTIC TESTSDTC P0135 (FO2H): FRONT O2S HEATER CIRCUIT MALFUNCTION
1998 Subaru Forester L, Standard
DTC P0135 (FO2H): Front O2S Heater Circuit Malfunction
1998 Subaru Forester L, StandardSECTION DTC P0135 (FO2H): Front O2S Heater Circuit Malfunction
- Connect scan tool and check for DTC(s). If DTC P0135 is only code set, go to next step. If DTC P0141 also exists, turn ignition off. Disconnect PCM connector. Check resistance of harness between ground and PCM connector terminal No. 42 (Black/White wire on Forester and Legacy, or Black/Yellow wire on Impreza). If resistance is 5 ohms or less, repair poor contact in PCM connector. If resistance is more than 5 ohms, check for an open in harness or poor contact in front O2S.
- Start engine. Observe front O2S heater current display. If value is more than 0.2 amp, stop engine and repair O2S connector and/or PCM connector. If value is 0.2 amp or less, leave engine running. Using voltmeter connected to ground, backprobe PCM connector terminal No. 38 (Brown/Black wire on Forester and Impreza, or White wire on Legacy). If voltage is one volt or less, go to step 4). If voltage is more than one volt, leave voltmeter connected and go to next step.
- Wiggle wiring and connector of PCM. If voltage varies more than one volt, repair poor connection at PCM connector. If wiggling wiring and connector does not change reading, disconnect connector from front O2S and recheck voltage. If voltage is one volt or less, replace PCM. If voltage is always more than one volt, check for short to voltage in wiring between O2S connector and PCM connector. After repairing short, check PCM operation and replace if necessary.
- Turn ignition off. Disconnect front O2S connector. Turn ignition on. Measure voltage between ground and front O2S connector terminal No. 2 (Yellow/Red wire). If voltage is more than 10 volts, go to next step. If voltage is 10 volts or less, check power supply line between main relay and front O2S for an open, or for poor terminal contact at main relay or O2S connector.
- Turn ignition off. Measure resistance of front O2S between connector terminals No. 2 (Yellow/Red wire) and No. 1 (Brown/Black wire on Forester and Impreza, or White wire on Legacy). If resistance is more than 30 ohms, replace O2S. If resistance is 30 ohms or less, check for an open in wiring between front O2S connector and PCM connector, or for poor contact in front O2S connector or PCM connector.
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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.