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 P0139 (RO2_R): REAR O2S CIRCUIT SLOW RESPONSE
1998 Subaru Forester L, Standard
DTC P0139 (RO2_R): Rear O2S Circuit Slow Response
1998 Subaru Forester L, StandardSECTION DTC P0139 (RO2_R): Rear O2S Circuit Slow Response
If DTC P0136 exists, diagnose DTC P0136 first. Clear DTCs and retest. If DTC P0139 is only code set, replace rear O2S and retest system.
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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.