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: General Description
The secondary heated oxygen sensor (secondary HO2S (sensor 2)) detects the oxygen content in the exhaust gas downstream of the three way catalytic converter (TWC) during stoichiometric air/fuel ratio feedback control. The powertrain control module (PCM) controls the air/fuel ratio with the air/fuel ratio (A/F) sensor (sensor 1) output voltage to optimize the TWC's efficiency. When the secondary HO2S (sensor 2) does not change during lean air/fuel ratio when the throttle is closed, after full current is applied to the secondary HO2S (sensor 2) heater, and output remains within the specified range continuously, the PCM detects a malfunction and stores a DTC.
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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.