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DTC P1238: Mass Airflow Sensor Plausibility (torque monitor)
 
 
 
 
TECHNICAL DESCRIPTION
Compare the actual measurement of volumetric efficiency by a mass airflow sensor signal with volumetric efficiency estimated from a throttle position sensor (main or sub) signal.
 
 
MONITOR EXECUTION
Continuous
 
 
MONITOR EXECUTION CONDITIONS (Other monitor and Sensor)
Other Monitor (There is no temporary DTC set 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
  • The volumetric efficiency estimated by the throttle position sensor (main) subtracted from the actual volumetric efficiency is more than 0 percent. Or, the volumetric efficiency is less than 60 percent.
  • Engine speed is more than 750 r/min.
Judgment Criterion
  • For 4.5 second, the difference between the actual volumetric efficiency and the volumetric efficiency estimated by the throttle position sensor (main) is more than 35 percent.
Check Conditions
  • The volumetric efficiency estimated by the throttle position sensor (sub) subtracted from the actual volumetric efficiency is more than 0 percent. Or, the volumetric efficiency is less than 60 percent.
  • Engine speed is more than 750 r/min.
Judgment Criterion
  • For 4.5 second, the difference between the actual volumetric efficiency and the volumetric efficiency estimated by the throttle position sensor (sub) is more than 35 percent.
 
 
FAIL-SAFE AND BACKUP FUNCTION
  • Throttle opening degree position is in default position.
 
 
OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle Pattern 17 .
 
 
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
  • Mass airflow sensor system failed.
  • Intake system vacuum leak.
  • 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, read the diagnostic trouble code (DTC).
cautionTo 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) Turn the ignition switch to the "ON" position.
(3) Read the DTC.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is the diagnostic trouble code other than P1238 set?
Refer to, Diagnostic Trouble Code Chart .
Go to Step 2.
 
STEP 2. Check for intake system vacuum leak.
Are there any abnormalities?
Repair it. Then go to Step 5.
Go to Step 3.
 
STEP 3. Using scan tool MB991958, check data list item 10: Mass Airflow Sensor.
(1) Start the engine and run at idle.
(2) Set scan tool MB991958 to the data reading mode for item 10, Mass Airflow Sensor.
(3) Warm up the engine to normal operating temperature: 80°C to 95°C (176°F to 203°F).
  • The standard value during idling should be between 1,350 and 1,670 millivolts.
  • When the engine is revved, the output voltage should increase according to the increase in engine speed.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is the sensor operating properly?
Go to Step 4.
Refer to, DTC P0102 - Mass Airflow Circuit Low Input , DTC P0103 - Mass Airflow Circuit High Input .
 
STEP 4. Check the trouble symptoms.
(1) Carry out a test drive with the drive cycle pattern. Refer to Diagnostic Function - OBD-II Drive Cycle Pattern 17 .
(2) Check the diagnostic trouble code (DTC).
Is DTC P1238 set?
Replace the ECM (Refer to, Removal and Installation ). Then go to Step 5.
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 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 17 .
(2) Check the diagnostic trouble code (DTC).
Is DTC P1238 set?
Retry the troubleshooting.
The inspection is complete.