P0138

DTC P0138: Cylinder 1, 4 Heated Oxygen Sensor (rear) Circuit High Voltage

Cylinder 1, 4 Heated Oxygen Sensor (rear) Circuit Part 1:

Cylinder 1, 4 Heated Oxygen Sensor (rear) Circuit Part 2:

CIRCUIT OPERATION
- A voltage corresponding to the oxygen concentration in the exhaust gas is sent to the ECM or the PCM (terminal No. 97) from the output terminal (terminal No. 4) of the cylinder 1, 4 heated oxygen sensor (rear).
- Terminal No. 2 of the cylinder 1, 4 heated oxygen sensor (rear) is grounded with ECM or PCM (terminal No. 106).
- The ECM or the PCM applies an offset voltage of 0.5 volt to terminal No. 2 of the cylinder 1, 4 heated oxygen sensor (rear).

TECHNICAL DESCRIPTION
- The output signal of the cylinder 1, 4 heated oxygen sensor (front) is compensated by the output signal of the cylinder 1, 4 heated oxygen sensor (rear).
- The ECM or the PCM checks for the cylinder 1, 4 heated oxygen sensor (rear) out- put voltage.

DESCRIPTIONS OF MONITOR METHODS
Cylinder 1, 4 heated oxygen sensor (rear) output voltage is over specified range.

MONITOR EXECUTION
Continuous

MONITOR EXECUTION CONDITIONS (Other monitor and Sensor)

Other Monitor (There is no temporary DTC stored in memory for the item monitored below)
- Heated oxygen sensor heater (front) monitor
- Heated oxygen sensor heater (rear) monitor
- Air/fuel ratio feedback monitor

Sensor (The sensor below is determined to be normal)
- Mass airflow sensor
- Engine coolant temperature sensor
- Intake air temperature sensor
- Barometric pressure sensor

DTC SET CONDITIONS

Logic Flow Chart:

Logic Flow Chart

Check Conditions
- 2 seconds or more have passed since the engine starting sequence was completed.
- Heated oxygen sensor offset voltage is between 0.4 and 0.6 volt.

Judgement Criterion
- Cylinder 1, 4 heated oxygen sensor (rear) output voltage has continued to be 1.8 volts or higher for 2 seconds.

OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function OBD-II Drive Cycle Pattern 22. OBD-II Drive Cycle

TROUBLESHOOTING HINTS (The most likely causes for this code to be set are)
- Short circuit in cylinder 1, 4 heated oxygen sensor (rear) output line.
- Connector damage.
- ECM failed.
- PCM failed.

DIAGNOSIS

Required Special Tools:
- MB991958: Scan tool (M.U.T.-III Sub Assembly)
- MB991824: V.C.I.
- MB991827: USB Cable
- MB991910: Main Harness A

STEP 1. Check harness connector B-37 at cylinder 1, 4 heated oxygen sensor (rear) and harness connector B-21 at ECM or PCM for damage.

Q: Is the harness connector in good condition?

YES: Go to Step 2.

NO: Repair or replace it. Then go to Step 3.

STEP 2. Check for short circuit to power supply between cylinder 1, 4 heated oxygen sensor (rear) connector B-37 (terminal No. 4) and ECM or PCM connector B-21 (terminal No. 97).

Q: Is the harness wire in good condition?

YES: Replace the ECM or the PCM. When the ECM or the PCM is replaced, register the encrypted code. Refer to Encrypted Code Registration Criteria Table. Then go to Step 3.

NO: Repair it. Then go to Step 3.

STEP 3. Test the OBD-II drive cycle.

1. Carry out a test drive with the drive cycle pattern. Refer to Diagnostic Function OBD-II Drive Cycle Pattern 22. OBD-II Drive Cycle
2. Check the diagnostic trouble code (DTC).

Q: Is DTC P0138 set?

YES: Retry the troubleshooting.

NO: The inspection is complete.