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

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 from the air/fuel ratio (A/F) sensor (sensor 1) output voltage to optimize the 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 SO2 SG 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
5.0 seconds or more
DTC Type
Two drive cycles, MIL on

Enable Conditions
Condition
Minimum
Maximum
Elapsed time that secondary HO2S activity is not monitored after starting the engine
15 seconds
–
Fuel feedback
Closed loop

Malfunction Threshold
Either of the condition continues for at least 5.0 seconds:
The secondary HO2S (sensor 2) output voltage (voltage difference between SO2 and SO2 SG terminals) is 3.28 V or more.
The SO2 SG terminal voltage is 3.49 V or more.

Possible Cause
NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
Secondary HO2S (sensor 2) internal circuit failure
Secondary HO2S (sensor 2) output rich side stuck
Secondary HO2S (sensor 2) SO2 SG line short to power
Secondary HO2S (sensor 2) SO2 SG line open

Confirmation Procedure
Operating Condition

1.
Start the engine, and let it idle until the radiator fan comes on.
2.
Let the engine idle for at least 5 seconds.
3.
When the diagnosis does not finish at idle, drive immediately at a steady engine speed [ENGINE SPEED] between 1,500 - 3,000 rpm for at least 5 seconds.
When freeze data is stored, drive the vehicle under those conditions instead of Driving Pattern step 3.
Drive the vehicle in this manner only if the traffic regulations and ambient conditions allow.

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.