P0138

Diagnostic Chart:

Schematic:

The heated oxygen (HO2S) bank 1 sensor 2 is threaded into the front exhaust pipe directly behind the cylinder 1, 3, 5 pre-converter (cylinders 1, 3, 5 are closest to front of dash). The sensor is used by the ECM to determine the oxygen storage capability of the pre-converter, fuel trim biasing (time held lean or rich) and certain techniques used to diagnose DTCs associated with the pre-catalyst oxygen sensor. The sensor is an electrical source that responds to oxygen content out of the pre-converter. When the sensor reaches approximately 316°C (600°F), it is fully functional and produces a voltage based on the difference in oxygen between the atmosphere and exhaust gas. The ECM sends a 450 mV bias voltage on the signal line, which is pulled up through high resistance. When the HO2S bank 1 sensor 2 is cold, it produces no voltage and has extremely high internal resistance. The internal resistance of the sensor is much greater than the resistance of the bias pull-up resistor. However, when the sensor heats up, it produces voltage that overrides the bias voltage. Under normal conditions, low sensor voltage means high oxygen content/lean air-fuel mixture and vice versa. Normal sensor readings will be around 500-800 mV under steady cruise. DTC P0138 will set if the HO2S bank 1 sensor 2 signal is too high for a certain length of time; OR if the HO2S bank 1 sensor 2 does not go under 600 mV at least once within a certain amount of time when the HO2S bank 1 sensor 2 is being used for fuel trim biasing or catalytic converter monitoring.

DTC PARAMETERS
DTC P0138 will set if HO2S bank 1 sensor 2 signal is greater than 1080 mV when:
^ Condition exists for longer than 5 seconds
^ Exhaust pipe moisture has evaporated (calculated engine run time between 0 and 200 seconds), then engine must run for longer than 80 seconds
^ Calculated exhaust gas temperature is less than 800°C (1472°F)
^ Main relay voltage is greater than 10.5 volts

OR

HO2S bank 1 sensor 2 signal is greater than 202 mV in decel fuel cutoff (DFCO) when:
^ Condition exists for longer than 2 seconds
^ DFCO mode has been active for 0.5 seconds

OR

HO2S bank 1 sensor 2 signal does not go under 600 mV at least once in 400 seconds when the ECM has one time invoked a lean condition when:
^ HO2S bank 1 sensor 2 loop status is closed
^ Engine air flow is greater than 9.72 g/s
^ ECM has determined that a rich or lean engine running condition is NOT present (long term fuel trim is OK)
^ EVAP leak diagnostics are not running
^ No CAM, EVAP purge solenoid, fuel trim, HO2S bank 1 sensor 2 heater or misfire DTCs have been set

DTC P0138 diagnostic runs continuously once the above conditions have been met.

P0138 is a type B DTC.

DIAGNOSTIC AIDS
To locate an intermittent problem, use Scan tool to monitor HO2S BANK 1 SENSOR 2 voltage with ignition On, engine Off after the engine has cooled down. Wiggling wires while watching for a change from bias voltage (450 mV) may locate the area where a short to voltage in the wiring may exist.

The Scan tool contains the special function BANK 1 SENSOR 2 OSCILLATION TEST. This test will force the ECM to introduce an oscillating air/fuel ratio, while it monitors the performance of the HO2S bank 1 sensor 2. DTC P0138 will reset if the fault is currently present.