Advanced Diagnostics

DTC P0138: Rear Secondary Heated oxygen Sensor (Secondary HO2S (Bank 1, Sensor 2)) Circuit High Voltage




General Description
The rear secondary heated oxygen sensor (HO2S) (bank 1, sensor 2) detects the oxygen content in the exhaust gas downstream of the three way catalytic converter (TWO) during stoichiometric air/fuel ratio feedback control. The rear secondary HO2S controls the air/fuel ratio from the rear A/F sensor output voltage to optimize TWO efficiency.

After current is applied to the rear secondary HO2S heater, if the rear secondary HO2S output continues high (rich) exceeding the upper limit used during feedback control, a malfunction is detected and a DTC is stored.




Monitor Execution, Sequence, Duration, DTC Type, OBD Status




Enable Conditions

Malfunction Threshold
The rear secondary HO2S output voltage is 1.270 V or more for at least 5 seconds.

Driving Pattern
1. Start the engine. Let it idle until the radiator fan comes on.
2. Then, drive immediately at a steady engine speed between 1,500 - 3,000 rpm for at least 1 minute.

- Drive the vehicle in this manner only if the traffic regulations and ambient conditions allow.

Diagnosis Details

Conditions for illuminating the MIL
When a malfunction is detected during the first drive cycle, a Temporary DTC is stored in the ECM/PCM memory. If the malfunction recurs during the next (second) drive cycle, the MIL comes on and the DTC and the freeze frame data are stored.

Conditions for clearing the MIL
The MIL will be cleared if the malfunction does not recur during three consecutive trips in which the diagnostic runs.

The MIL, the DTC, the Temporary DTC, and the freeze frame data can be cleared by using the scan tool Clear command or by disconnecting the battery.