Code No. P0138: Oxygen Sensor (Rear) Circuit High Voltage








OPERATION



  • The sensor signal is inputted to the engine-ECU (terminal No. 40) from the oxygen sensor (rear) output terminal (terminal No. 3).
  • The oxygen sensor (rear) (terminal No. 4) is earthed with engine-ECU (terminal No. 41).

FUNCTION



  • The oxygen sensor (rear) converts the concentration of oxygen in the exhaust emission into a voltage and inputs the signal to the engine-ECU.
  • When the air-fuel ratio is richer than the theoretical air-fuel ratio, the oxygen sensor (rear) outputs a voltage of about 1 V. When it is leaner than the theoretical air-fuel ratio, it outputs a voltage of about 0 V.
  • In response to the signal, the engine-ECU controls the fuel injection amount so that the air-fuel ratio can be equivalent to the theoretical air-fuel ratio.

TROUBLE JUDGMENT


Check Conditions
  • 2 seconds or more have passed since the engine starting sequence was completed.
  • Oxygen sensor offset voltage is 0.4 - 3.8 V.
Judgement Criterion
  • Oxygen sensor (rear) output voltage has continued to be 1.2 V or higher for 2 seconds.

PROBABLE CAUSES



  • Failed oxygen sensor (rear)
  • Short circuit in oxygen sensor (rear) heater circuit or loose connector contact
  • Failed engine-ECU

DIAGNOSIS PROCEDURE

STEP 1. M.U.T.-III data list

  • Refer to Data List Reference Table .

  1. Item AD: Oxygen sensor (rear)

Q. Is the check result normal?
Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).
Go to Step 2 .

STEP 2. Connector check: D-31 oxygen sensor (rear) connector

Q. Is the check result normal?
Go to Step 3 .
Repair or replace the connector.

STEP 3. Connector check: B-29 engine-ECU connector

Q. Is the check result normal?
Go to Step 4 .
Repair or replace the connector.

STEP 4. Check harness between D-31 (terminal No. 3) oxygen sensor (rear) connector and B-29 (terminal No. 40) engine-ECU connector.
note Before checking harness, check intermediate connectors A-13 and C-131, and repair if necessary.

  • Check output line for short circuit.

Q. Is the check result normal?
Go to Step 5 .
Repair the damaged harness wire.

STEP 5. Check harness between D-31 (terminal No. 4) oxygen sensor (rear) connector and B-29 (terminal No. 41) engine-ECU connector.
note Before checking harness, check intermediate connectors A-13 and C-131, and repair if necessary.

  • Check earthing line for short circuit.

Q. Is the check result normal?
Go to Step 6 .
Repair the damaged harness wire.

STEP 6. M.U.T.-III data list

  • Refer to Data List Reference Table .

  1. Item AD: Oxygen sensor (rear)

Q. Is the check result normal?
Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).
Replace the engine-ECU.