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.
DTC P0139: 1/2 O2 Sensor Slow Response: Testing
- Turn ignition on. Using scan tool, read DTCs. If GOOD TRIP is displayed and equal to zero, go to next step. If GOOD TRIP is not displayed and equal to zero, go to step 6 .
- Start engine. Check for exhaust leaks between engine and HO2S. If no leaks are present, go to next step. If any leaks are found, repair as necessary.
- With engine idling, measure voltage drop (backprobe connectors with voltmeter) of signal circuit (Tan/White wire) between 1/2 HO2S harness connector and terminal No. 51 at PCM C2 harness connector. See Figure . PCM is located in engine compartment, between air cleaner and radiator. If voltage drop is less than .1 volt, go to next step. If voltage drop is .1 volt or more, repair high resistance in signal circuit.
- With engine idling, measure voltage drop (backprobe connectors with voltmeter) of sensor ground circuit (Black/Orange wire) between 1/2 HO2S harness connector and terminal No. 27 at PCM C1 harness connector. If voltage drop is less than .1 volt, go to next step. If voltage drop is .1 volt or more, repair high resistance in sensor ground circuit.
- If no other possible causes are remaining, replace PCM.
- Conditions that set DTC are not present at this time. Perform the following to help identify intermittent condition:
- With engine running at normal operating temperature, wiggle related wiring harness while monitoring related parameters on scan tool. If voltage changes or DTC sets while wiggling wiring harness, repair connectors or wiring harness where wiggling caused voltage to change or DTC to set.
- Review freeze frame information on scan tool. Duplicate conditions present when DTC was set.
- Refer to any Technical Service Bulletins (TSBs) that may apply.
- Visually inspect related wiring harness. Check for chafed, pierced, pinched or partially broken wires.
- Visually inspect related wiring harness connectors. Check for broken, bent, pushed out or corroded terminals.
If a problem is found, repair as necessary. If no problems are found, test is complete.
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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.
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