P0138

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














CIRCUIT OPERATION

- A voltage corresponding to the oxygen concentration in the exhaust gas is sent to the ECM [M/T] or the PCM [A/T] (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 [M/T] or PCM [A/T] (terminal No. 106).
- The ECM [M/T] or the PCM [A/T] 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 [M/T] or the PCM [A/T] checks for the cylinder 1, 4 heated oxygen sensor (rear) output 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






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 23 Monitors, Trips, Drive Cycles and Readiness Codes .

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. [M/T]
- PCM failed. [A/T]

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 [M/T] or PCM [A/T] for damage.









Q. Is the harness connector in good condition?

YES Go to Step 2.

NO Repair or replace it. Harness Connector Inspection.
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 [M/T] or PCM [A/T] 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. Immobilizer System.
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 23 Monitors, Trips, Drive Cycles and Readiness Codes.

(2)Check the diagnostic trouble code (DTC).

Q. Is DTC P0138 set?

YES Retry the troubleshooting.
NO The inspection is complete.