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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 heater (front) current or voltage
is out of specified range.
MONITOR EXECUTION
MONITOR EXECUTION CONDITIONS (Other monitor and Sensor)
Other Monitor (There is no temporary DTC stored in memory
for the item monitored below)
Sensor (The sensor below is determined to be normal)
DTC SET CONDITIONS
Logic Flow Chart
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 has continued to be lower 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 has continued to be lower than 2.0
volts for 2 seconds.
OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function -
OBD-II Drive Cycle -
Pattern
2 
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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.
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Required Special Tools:
- MB991658: Test Harness
- MB992110: Power Plant ECU Check Harness
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STEP 1. Check harness connector C-44 at the heated oxygen
sensor (front) for damage.
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Q.
Is the harness connector in good condition?
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 Go to Step 2.
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 Repair or replace it. Refer to GROUP 00E, Harness Connector Inspection .
Then go to Step 12.
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STEP 2. Check the heated oxygen sensor (front).
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(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.
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(2)Measure the resistance between heated oxygen sensor connector terminal No. 1 and terminal
No. 2.
Standard value: 4.5 -
8.0 Ω
[at 20°C (68°F)]
Q.
Is the measured resistance between 4.5 and 8.0 Ω
[at 20°C
(68°F)]?
Go to Step 3.
Replace the heated oxygen sensor (front). Then go to Step 12.
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STEP 3. Measure the power supply voltage at heated oxygen sensor (front)
harness side connector C-44.
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(1)Disconnect the connector C-44 and measure at the harness side.
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(2)Turn the ignition switch to the "ON" position.
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(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?
Go to Step 5.
Go to Step 4.
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STEP 4. Check harness connector A-30X at the MFI relay for damage.
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Q.
Is the harness connector in good condition?
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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 12.
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Repair or replace it. Refer to GROUP 00E, Harness Connector Inspection .
Then go to Step 12.
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STEP 5. Measure the power supply voltage at ECM connector B-108 by
using power plant ECU check harness special tool MB992110.
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(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.
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(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?
Go to Step 8.
Go to Step 6.
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STEP 6. Check harness connector B-108 at ECM for damage.
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Q.
Is the harness connector in good condition?
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Go to Step 7.
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Repair or replace it. Refer to GROUP 00E, Harness Connector Inspection .
Then go to Step 12.
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STEP 7. Check for open circuit or short circuit to ground between
heated oxygen sensor (front) connector C-44 (terminal No. 2) and ECM connector B-108 (terminal
No. 34).
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Check harness after checking intermediate connector A-10. If intermediate connector is
damaged, repair or replace it. Refer to GROUP 00E, Harness Connector Inspection . Then
go to Step 12.
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Q.
Is the harness wire in good condition?
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Replace the ECM. When the ECM is replaced, register the ID code. Refer to GROUP
42B, Diagnosis -
ID Code Registration Judgment Table <Vehicles with KOS> or
GROUP 42C, Diagnosis -
ID Codes Registration Judgment Table <Vehicles with WCM> .
Then go to Step 12.
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Repair it. Then go to Step 12.
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STEP 8. Check harness connector B-108 at ECM for damage.
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Q.
Is the harness connector in good condition?
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Go to Step 9.
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Repair or replace it. Refer to GROUP 00E, Harness Connector Inspection .
Then go to Step 12.
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STEP 9. Check for harness damage between MFI relay connector A-30X
(terminal No. 2) and heated oxygen sensor (front) connector C-44 (terminal No. 1).
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Q.
Is the harness wire in good condition?
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Go to Step 10.
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Repair it. Then go to Step 12.
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STEP 10. Check for harness damage between heated oxygen sensor (front)
connector C-44 (terminal No. 2) and ECM connector B-108 (terminal No. 34).
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Check harness after checking intermediate connector A-10. If intermediate connector is
damaged, repair or replace it. Refer to GROUP 00E, Harness Connector Inspection . Then
go to Step 12.
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Q.
Is the harness wire in good condition?
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Go to Step 11.
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Repair it. Then go to Step 12.
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STEP 11. Check the trouble symptoms.
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(1)Carry out a test drive with the drive cycle pattern. Refer to Diagnostic Function -
OBD-II
Drive Cycle -
Pattern 2 .
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(2)Check the diagnostic trouble code (DTC).
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Replace the ECM. When the ECM is replaced, register the ID code. Refer to GROUP
42B, Diagnosis -
ID Code Registration Judgment Table <Vehicles with KOS> or
GROUP 42C, Diagnosis -
ID Codes Registration Judgment Table <Vehicles with WCM> .
Then go to Step 12.
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It can be assumed that this malfunction is intermittent. Refer to GROUP 00, How
to Use Troubleshooting/Inspection Service Points -
How to Cope with Intermittent
Malfunctions .
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STEP 12. Test the OBD-II drive cycle.
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(1)Carry out a test drive with the drive cycle pattern. Refer to Diagnostic Function -
OBD-II
Drive Cycle -
Pattern 2 .
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(2)Check the diagnostic trouble code (DTC).
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Retry the troubleshooting.
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The inspection is complete.
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