DTC P0138 (2020 2021)

DTC P0138:  Secondary Heated Oxygen Sensor (Secondary HO2S (Sensor 2)) Circuit High Voltage

General Description 

Courtesy of HONDA, U.S.A., INC.

Courtesy of HONDA, U.S.A., INC.

The secondary heated oxygen sensor (secondary HO2S (sensor 2)) detects the oxygen content in the exhaust gas downstream of the warm up three way catalytic converter (WU-TWC) during stoichiometric air/fuel ratio feedback control. The powertrain control module (PCM) controls the air/fuel ratio from the air/fuel ratio (A/F) sensor (sensor 1) output voltage to optimize the WU-TWC efficiency. When power is applied to the secondary HO2S (sensor 2) heater, if the secondary HO2S (sensor 2) output voltage is high or the SO2SG terminal voltage is high during feedback control, the PCM detects a malfunction and stores a DTC.

Monitor Execution, Sequence, Duration, DTC Type 

Execution	Continuous
Sequence	None
Duration	5 seconds or more
DTC Type	Two drive cycles, MIL on

Enable Conditions 

Condition
State of the engine	Running

Malfunction Threshold 

Either of the condition continues for at least 5 seconds: 

• The secondary HO2S (sensor 2) output voltage (voltage difference between SO2 and SO2SG terminals) [HO2S S2] is 3.28 V or more.
• The SO2SG terminal voltage is 3.49 V or more.

[ ]: HDS Parameter

Possible Cause 

Diagnosis Details 

Conditions for setting the DTC 

When a malfunction is detected during the first drive cycle, a Pending DTC is stored in the PCM memory. If the malfunction returns in the next (second) drive cycle, the MIL comes on and a Confirmed DTC and the freeze data are stored.

Conditions for clearing the DTC 

The MIL is cleared if the malfunction does not return in three consecutive trips in which the diagnostic runs. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool Clear command or by disconnecting the 12 volt battery.