Inspection Procedure 4

INSPECTION PROCEDURE 4: The Malfunction Indicator Lamp (SERVICE ENGINE SOON) Remains Illuminated and Never Goes Out

Malfunction Indicator Lamp (Service Engine Soon) Circuit Part 1:

Malfunction Indicator Lamp (Service Engine Soon) Circuit Part 2:

CIRCUIT OPERATION
- The malfunction indicator lamp (SERVICE ENGINE SOON) power is supplied from the ignition switch.
- The ECM controls the ground of the malfunction indicator lamp (SERVICE ENGINE SOON) by turning the power transistor in the ECM ON and OFF.

COMMENT
- In cases such as the above, the cause is probably that the ECM is detecting a problem in a sensor or actuator, or that one of the malfunctions listed at right has probably occurred.

TROUBLESHOOTING HINTS (The most likely causes for this case:)
- Short-circuit between the malfunction indicator lamp (SERVICE ENGINE SOON) and ECM.
- Malfunction of the ECM.

DIAGNOSIS

Required Special Tools:
- MB991958: Scan Tool (M.U.T.-III Sub Assembly)
- MB991824: V.C.I.
- MB991827: USB Cable
- MB991911: Main Harness B5

STEP 1. Using scan tool MB991958, read the diagnostic trouble code (DTC).

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. Turn the ignition switch to the "ON" position.
3. Read the DTC.
4. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is DTC set?

YES: Refer to, Diagnostic Trouble Code Chart. Diagnostic Trouble Code Descriptions

NO: Go to Step 2.

STEP 2. Check for continuity at combination meter harness side connector C-01.




1. Disconnect the connector C-01 and measure at the harness side.




2. Check for the continuity between terminal No.17 and ground.
- Should be open loop.

Q: Does continuity exist?

YES: Replace the ECM. Then confirm that the malfunction symptom is eliminated.

NO: Check harness connectors C-124 at the intermediate connector for damage, and repair or replace as required. If intermediate connector is in good condition, repair harness wire between combination meter connector C-01 (terminal No.17) and ECM connector C-121 (terminal No.22) because of short circuit to ground. Then confirm that the malfunction symptom is eliminated.