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DTC P0031: Linear Air-Fuel Ratio Sensor Heater Control Circuit Low
 
 
 
 
CIRCUIT OPERATION
  • Power is supplied from the MFI relay to the linear air-fuel ratio sensor heater.
  • The ECM terminal LHFL controls continuity to the linear air-fuel ratio sensor 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
  • Linear air-fuel ratio sensor 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
Logic Flow Chart
 
 
Check Conditions
  • More than 2 seconds have passed since the engine starting sequence was completed.
  • While the linear air-fuel ratio sensor heater is on.
  • Battery positive voltage is between 11 and 16.5 volts.
Judgment Criterion
  • The linear air-fuel ratio sensor heater current has continued to be lower than average 0.17 ampere for 2 seconds.
Check Conditions
  • More than 2 seconds have passed since the engine starting sequence was completed.
  • While the linear air-fuel ratio sensor heater is off.
  • Battery positive voltage is between 11 and 16.5 volts.
Judgment Criterion
  • The linear air-fuel ratio sensor heater voltage has continued to be lower than average 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 .
 
 
TROUBLESHOOTING HINTS (The most likely causes for this code to be stored are: )
  • Open or shorted linear air-fuel ratio sensor heater circuit, harness damage or connector damage.
  • Linear air-fuel ratio sensor heater failed.
  • ECM failed.

 
 
DIAGNOSIS
Required Special Tool:
  • MB992110: Power Plant ECU Check Harness
STEP 1. Check the linear air-fuel ratio sensor.
(1) Disconnect the linear air-fuel ratio sensor connector and measure at linear air-fuel ratio sensor side.
(2) Measure the resistance between linear air-fuel ratio sensor connector terminal power supply line and terminal LHFL.
Standard value: 2.3 - 4.2 ohms [at 20°C (68°F)]
Is the measured resistance between 2.3 and 4.2 ohms [at 20°C (68°F)]?
Go to Step 2.
Replace the linear air-fuel ratio sensor. Then go to Step 7.
 
STEP 2. Measure the power supply voltage at linear air-fuel ratio sensor harness side connector.
(1) Disconnect the linear air-fuel ratio sensor connector and measure at the harness side.
(2) Turn the ignition switch to the "ON" position.
(3) Measure the voltage between terminal power supply line and ground.
  • Voltage should be battery positive voltage.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is battery positive voltage (approximately 12 volts) present?
Go to Step 3.
Repair or replace the connector(s), or check and repair the harness of short to ground and open circuit in power supply line between MFI relay connector and linear air-fuel ratio sensor connector. Then go to Step 7.
 
STEP 3. Measure the power supply voltage at ECM connector 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 LHFL and ground.
  • Voltage should be battery positive voltage.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is battery positive voltage (approximately 12 volts) present?
Go to Step 4.
Repair or replace the connector(s), or check and repair the harness of short to ground and open circuit in LHFL line between linear air-fuel ratio sensor connector and ECM connector. Then go to Step 7.
 
STEP 4. Check of harness damage in power supply line between MFI relay connector and linear air-fuel ratio sensor connector.
Is the harness wire in good condition?
Go to Step 5.
Repair the connector(s) or wiring harness. Then go to Step 7.
 
STEP 5. Check of harness damage in LHFL line between linear air-fuel ratio sensor connector and ECM connector.
Is the harness wire in good condition?
Go to Step 6.
Repair the connector(s) or wiring harness. Then go to Step 7.
 
STEP 6. 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 .
(2) Check the diagnostic trouble code (DTC).
Is DTC P0031 stored?
Replace the ECM (Refer to Removal and Installation ). Then go to Step 7.
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 .
 
STEP 7. 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 .
(2) Check the diagnostic trouble code (DTC).
Is DTC P0031 stored?
Retry the troubleshooting.
The inspection is complete.