April 5, 2026: LEMON Manuals is launched! Read the announcement.

DTC P0138

DTC P0138

HO2S circuit high input

DETECTION CONDITION

The PCM monitors the input voltage from the HO2S. If the input voltage from the HO2S is above 1.2 V or HO2S bias voltage is above 1.7 V for 5 s, the PCM determines that the circuit input is high.

Diagnostic support note

This is a continuous monitor (A/F sensor, HO2S).
The MIL illuminates if the PCM detects the above malfunction condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM.
PENDING CODE is available if the PCM detects the above malfunction condition during the first drive cycle.
FREEZE FRAME DATA (Mode 2/Mode 12) is available.
The DTC is stored in the PCM memory.

POSSIBLE CAUSE

HO2S connector or terminals malfunction
PCM connector or terminals malfunction
Short to power supply in wiring harness between HO2S terminal A and PCM terminal 2Q
Short to power supply in wiring harness between HO2S terminal B and PCM terminal 2BH
HO2S malfunction
PCM malfunction

G07878111

DIAGNOSTIC PROCEDURE

STEP	INSPECTION		ACTION
1	VERIFY FREEZE FRAME DATA (MODE 12) AND DIAGNOSTIC MONITORING TEST RESULTS HAVE BEEN RECORDED Have the FREEZE FRAME DATA (Mode 12) and DIAGNOSTIC MONITORING TEST RESULTS (A/F sensor, HO2S related) been recorded?	Yes	Go to the next step.
1		No	Record the FREEZE FRAME DATA (Mode 12) and DIAGNOSTIC MONITORING TEST RESULTS on the repair order, then go to the next step.
2	VERIFY RELATED REPAIR INFORMATION AVAILABILITY Verify related Service Bulletins and/or on-line repair information availability. Is any related repair information available?	Yes	Perform repair or diagnosis according to the available repair information. If the vehicle is not repaired, go to the next step.
2		No	Go to the next step.
3	INSPECT HO2S CONNECTOR FOR POOR CONNECTION Turn the ignition switch off. Disconnect the HO2S connector. Inspect for poor connection (such as damaged/pulled-out pins, corrosion). Is there any malfunction?	Yes	Repair or replace the connector and/or terminals, then go to Step 7.
3		No	Go to the next step.
4	INSPECT PCM CONNECTOR FOR POOR CONNECTION Turn the ignition switch off. Disconnect the PCM connector. Inspect for poor connection (such as damaged/pulled-out pins, corrosion). Is there any malfunction?	Yes	Repair or replace the connector and/or terminals, then go to Step 7.
4		No	Go to the next step.
5	INSPECT HO2S CIRCUIT FOR SHORT TO POWER SUPPLY HO2S and PCM connectors are disconnected. Turn the ignition switch to the ON position (engine off). Measure the voltage between the following circuits: HO2S terminal A (wiring harness-side) and body ground HO2S terminal B (wiring harness-side) and body ground Is the voltage more than 1.2 V?	Yes	Repair or replace the wiring harness for a possible short to power supply, then go to Step 7.
5		No	Go to the next step.
6	INSPECT HO2S Inspect the HO2S. (See HEATED OXYGEN SENSOR (HO2S) INSPECTION ). Is there any malfunction?	Yes	Replace the HO2S, then go to the next step. (See HEATED OXYGEN SENSOR (HO2S) REMOVAL/INSTALLATION ).
6		No	Go to the next step.
7	VERIFY TROUBLESHOOTING OF DTC P0138 COMPLETED Make sure to reconnect all disconnected connectors. Clear the DTC from the PCM memory using the M-MDS. (See AFTER REPAIR PROCEDURE ). Perform the KOER self test. (See KOEO/KOER SELF TEST ). Is the PENDING CODE same as the DTC present?	Yes	Replace the PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION ).
7		No	Go to the next step.
8	VERIFY AFTER REPAIR PROCEDURE Perform the "AFTER REPAIR PROCEDURE ". (See AFTER REPAIR PROCEDURE ). Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE ).
8		No	DTC troubleshooting completed.