P0138

Component Description
The rear heated oxygen sensor (Rear HO2S), after three way catalyst, monitors the oxygen level in the exhaust gas. Even if switching characteristics of the front heated oxygen sensor are shifted, the air fuel ratio is controlled to stoichiometric, by the signal from the rear heated oxygen sensor.

This sensor is made of ceramic zirconia. The zirconia generates voltage from approximately 1 V in richer conditions to 0 V in leaner conditions.

Under normal conditions the rear heated oxygen sensor is not used for engine control operation.




CONSULT-II Reference Value in Data Monitor Mode
Specification data are reference values.




ECM Terminals and Reference Value
Specification data are reference values and are measured between each terminal and 32 (ECM ground).







On Board Diagnosis Logic
The rear heated oxygen sensor has a much longer switching time between rich and lean than the front heated oxygen sensor. The oxygen storage capacity before the three way catalyst causes the longer switching time. To judge the malfunctions of rear heated oxygen sensor, ECM monitors whether the maximum voltage of the sensor is sufficiently high during various driving conditions such as fuel-cut.

DTC Confirmation Procedure

NOTE: If "DIAGNOSTIC TROUBLE CODE CONFIRMATION PROCEDURE" has been previously conducted, always turn ignition switch OFF and wait at least 5 seconds before conducting the next test.

TESTING CONDITION: Open engine hood before conducting following procedure.










With CONSULT-II
1. Start engine and warm it up to normal operating temperature.
2. Turn ignition switch OFF and wait at least 5 seconds.
3. Turn ignition switch ON.
4. Select "DATA MONITOR" mode with CONSULT-II.
5. Make sure the "COOLANT TEMP/S" is more then 70 °C (158 °F).
6. Select "RR O2 SEN P0138", of "REAR O2 SENSOR" in "DTC WORK SUPPORT" mode with CONSULT-II.
7. Start engine and follow the instructions of CONSULT-II.
8. Make sure the "OK" is displayed after touching "SELF-DIAG RESULTS".
If NG is displayed, refer to "DIAGNOSTIC PROCEDURE".
If "CAN NOT BE DIAGNOSED" is displayed, perform the following.
a. Stop engine and cool down "COOLANT TEMP/SE" to less then 70 °C (158 °F).
b. Turn ignition switch ON.
c. Select "DATA MONITOR" mode with CONSULT-II.
d. Perform from step 6) again when the "COOLANT TEMP/S" reach to 70 °C (158 °F).

Overall Function Check
Use this procedure to check the overall function of the rear heated oxygen sensor circuit. During this check, a 1st trip DTC might not be confirmed.

CAUTION: Always drive vehicle at a safe speed.

Without CONSULT-II
1. Start engine and drive vehicle at a speed of 70 km/h (43 MPH) for 2 consecutive minutes.
2. Stop vehicle with engine running.




3. Set voltmeter probes between ECM terminals 56 (Rear heated oxygen sensor signal) and ECM ground.
4. Check the voltage when revving engine up to 4,000 rpm under no load at least 10 times.
(Depress and release accelerator pedal as soon as possible.)
The voltage should be above 0.56 V at least once during this procedure.
If the voltage can be confirmed in step 4, step 5 is not necessary.

5. Keep vehicle at idling for 10 minutes, then check the voltage. Or check the voltage when coasting from 80 km/h (50 MPH) in 3rd gear position.
The voltage should be above 0.56 V at least once during this procedure.
6. If NG, go to "Diagnostic Procedure" below.

EC-RRO2-01:

Wiring Diagram

Diagnostic Procedure

Step 1:

Step 2:

Step 3 - 5:

Step 6:

Step 7 - 8:

Step 9 - 10: