Heated Oxygen Sensor (Front) Heater Control Circuit Low

DTC P0031: Heated Oxygen Sensor (Front) Heater Control Circuit Low










CIRCUIT OPERATION
- Power is supplied from the MFI relay (terminal No. 2) to the heated oxygen sensor (front) heater.
- The ECM (terminal No. 34) controls continuity to the heated oxygen sensor (front) heater by turning the power transistor in the ECM "ON" and "OFF".

TECHNICAL DESCRIPTION
- The ECM checks whether the heater current is within a specified range when the heater is energized.
- The ECM checks whether the heater voltage is within a specified range when the heater is not energized.

DESCRIPTIONS OF MONITOR METHODS
Heated oxygen sensor (front) heater current or voltage is out of 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)
- Not applicable

Sensor (The sensor below is determined to be normal)
- Not applicable

DTC SET CONDITIONS







Check Conditions
- More than 2 seconds have passed since the engine starting sequence was completed.
- While the heated oxygen sensor (front) heater is on.
- Battery positive voltage is between 11 and 16.5 volts.

Judgement Criterion
- The heated oxygen sensor (front) heater current is less than 0.17 ampere for 2 seconds.

Check Conditions
- More than 2 seconds have passed since the engine starting sequence was completed.
- While the heated oxygen sensor (front) heater is off.
- Battery positive voltage is between 11 and 16.5 volts.

Judgement Criterion
- The heated oxygen sensor (front) heater voltage is less than 2.0 volts for 2 seconds.

FAIL-SAFE AND BACKUP FUNCTION
- None

OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 2. OBD- II Drive Cycle

TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
- Open or shorted heated oxygen sensor (front) heater circuit, harness damage or connector damage.
- Heated oxygen sensor (front) heater failed.
- ECM 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
- MB991658: Test Harness
- MB992110: Power Plant ECU Check Harness

STEP 1. Check harness connector C-44 at heated oxygen sensor (front) for damage.

Q: Is the harness connector in good condition?

YES : Go to Step 2.

NO : Repair or replace it. Then go to Step 11.

STEP 2. Check the heated oxygen sensor (front).

1. Disconnect heated oxygen sensor (front) connector C-44 and connect test harness special tool, MB991658, to the connector on the heated oxygen sensor (front) side.







2. Measure the resistance between heated oxygen sensor connector terminal No. 1 and terminal No. 2.

Standard value: 4.5 - 8.0 ohms [at 20 °C (68 °F)]

Q: Is the measured resistance between 4.5 and 8.0 ohms [at 20 °C (68 °F)]?

YES : Go to Step 3.

NO : Replace the heated oxygen sensor (front). Then go Step 11.

STEP 3. Measure the power supply voltage at heated oxygen sensor (front) harness side connector C-44.

1. Disconnect the connector C-44 and measure at the harness side.
2. Turn the ignition switch to the "ON" position.







3. Measure the voltage between terminal No. 1 and ground.
- Voltage should be battery positive voltage.

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

Q: Is battery positive voltage (approximately 12 volts) present?

YES : Go to Step 5.

NO : Go to Step 4.

STEP 4. Check harness connector A-30X at MFI relay for damage.

Q: Is the harness connector in good condition?

YES : Repair harness wire between MFI relay connector A-30X (terminal No. 2) and heated oxygen sensor (front) connector C-44 (terminal No. 1) because of open circuit or short circuit to ground. Then go to Step 11.

NO : Repair or replace it. Then go to Step 11.

STEP 5. Check harness connector B-108 at ECM for damage.

Q: Is the harness connector in good condition?

YES : Go to Step 6.

NO : Repair or replace it. Then go to Step 11.

STEP 6. Measure the power supply voltage at ECM connector B-108 by using power plant ECU check harness special tool MB992110.







1. Disconnect all ECM connectors. Connect the power plant ECU check harness special tool MB992110 between the separated connectors.
2. Turn the ignition switch to the "ON" position.







3. Measure the voltage between terminal No. 34 and ground.
- Voltage should be battery positive voltage.

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

Q: Is battery positive voltage (approximately 12 volts) present?

YES : Go to Step 7.

NO : Check harness connector A-54 at intermediate connector for damage, and repair or replaces as required. If intermediate connector is in good condition, repair harness wire between heated oxygen sensor (front) connector C-44 (terminal No. 2) and ECM connector B-108 (terminal No. 34) because of open circuit or short circuit to ground. Then go to Step 11.

STEP 7. Check harness connector A-30X at MFI relay for damage.

Q: Is the harness connector in good condition?

YES : Go to Step 8.

NO : Repair or replace it. Then go to Step 11.

STEP 8. Check for harness damage between MFI relay connector A-30X (terminal No. 2) and heated oxygen sensor (front) connector C-44 (terminal No. 1).

Q: Is the harness wire in good condition?

YES : Go to Step 9.

NO : Repair it. Then go to Step 11.

STEP 9. Check for harness damage between heated oxygen sensor (front) connector C-44 (terminal No. 2) and ECM connector B-108 (terminal No. 34).

NOTE: Check harness after checking intermediate connector A-54. If intermediate connector is damaged, repair or replace it. Then go to Step 11.

Q: Is the harness wire in good condition?

YES : Go to Step 10.

NO : Repair it. Then go to Step 11.

STEP 10. Check the trouble symptoms.

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

Q: Is DTC P0031 set?

YES : Replace the ECM. Then go to Step 11.

NO : It can be assumed that this malfunction is intermittent. Refer to How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions. Troubleshooting Contents

STEP 11. 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 2. OBD- II Drive Cycle
2. Check the diagnostic trouble code (DTC).

Q: Is DTC P0031 set?

YES : Retry the troubleshooting.

NO : The inspection is complete.