P0138

DTC P0138: 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 (terminal No. 40) from the output terminal (terminal No.
3) of the heated oxygen sensor (rear).

- Terminal No. 4 of the heated oxygen sensor (rear) is grounded with ECM (terminal No. 41).

- The ECM applies an offset voltage of 0.5 V to terminal No. 4 of the heated oxygen sensor (rear).

TECHNICAL DESCRIPTION

- The output signal of the heated oxygen sensor (front) is compensated by the output signal of the heated oxygen sensor (rear).

- The ECM checks for the heated oxygen sensor (rear) output voltage.

DESCRIPTIONS OF MONITOR METHODS

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 (front) monitor

- Heated oxygen sensor (front) heater monitor

- Heated oxygen sensor (rear) heater monitor

- Heated oxygen sensor (front) inactive monitor

- Heated oxygen sensor offset voltage 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

- More than 2 seconds have passed since the engine starting sequence was completed.

- Heated oxygen sensor offset voltage is between 0.4 and 0.6 V.
Judgement Criterion

- Heated oxygen sensor (rear) output voltage is more than 1.8 V for 2 seconds.

FAIL-SAFE AND BACKUP FUNCTION

- None

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 heated oxygen sensor (rear) output line.

- Connector damage.

- ECM failed.

DIAGNOSIS

Required Special Tool:

- MB991958: Scan Tool (M.U.T.-III Sub Assembly)
- MB991824: V.C.I.

- MB991827: USB Cable

- MB991910: Main Harness A

STEP 1. Using scan tool MB991958, check data list item AD:
Heated Oxygen Sensor (rear).





CAUTION:
To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK"
(OFF) position before connecting or disconnecting scan tool MB991958.

(1)Connect scan tool MB991958 to the data link connector.
(2)Start the engine and run at idle.
(3)Set scan tool MB991958 to the data reading mode for item AD, Heated Oxygen Sensor
(rear).
1. Transaxle: 2nd speed

2. Drive with wide open throttle

3. Engine: 3,500 r/min or more

- The output voltages should be between 0.6 and 1.0 V.
(4)Turn
the ignition switch to the "LOCK" (OFF) position.

Q. Is the sensor operating properly?

YES It can be assumed that this malfunction is intermittent. How to Cope With Intermittent Malfunctions.

NO Go to Step 2.

STEP 2. Check harness connector D-40 at heated oxygen sensor (rear) and harness connector B-108 at ECM for damage.

Q. Are the harness connectors in good condition?

YES Go to Step 3.
NO Repair or replace them. Harness Connector Inspection.
Then go to Step 5.
STEP 3. Check for short circuit to power supply between heated oxygen sensor (rear) connector D-40 (terminal No. 3) and ECM connector B-108 (terminal No. 40).

NOTE:
Check harness after checking intermediate connectors A-54 and C-56. If intermediate connectors are damaged, repair or replace them. Harness Connector Inspection.
Then go to Step 5.

Q. Is the harness wire in good condition?

YES Go to Step 4.
NO Repair it. Then go to Step 5.
STEP 4. Using scan tool MB991958, check data list item AD: Heated Oxygen Sensor (rear).

(1)Start the engine and run at idle.

(2)Set scan tool MB991958 to the data reading mode for item AD, Heated Oxygen Sensor
(rear).
4. Transaxle: 2nd speed

5. Drive with wide open throttle

6. Engine: 3,500 r/min or more

- The output voltages should be between 0.6 and 1.0 V.

(3)Turn the ignition switch to the "LOCK" (OFF) position.

Q. Is the sensor operating properly?

YES It can be assumed that this malfunction is intermittent. How to Cope With Intermittent Malfunctions.
NO Replace the ECM (Refer to Removal and Installation Removal and Replacement).
Then go to Step 5.
STEP 5. 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.