Download In cases such as the above, the cause is probably the air/fuel mixture or idle air control motor. Other systems affecting idle quality include the ignition system and compression. TROUBLESHOOTING HINTS (The most likely causes for this case:) Malfunction of the ignition system. Malfunction of air/fuel ratio control system. Malfunction of the IAC system. Malfunction of the evaporative emission purge solenoid system. Poor compression pressure. Vacuum leak. Malfunction of the EGR solenoid system. STEP 1. Check if the battery terminal is disconnected. Q: Has the battery terminal been disconnected lately? YES: Start the engine and let it run at idle for approximate 10 minutes after engine warm up. Then, if a malfunction occurs, go to Step 2. NO: Go to Step 2. STEP 2. Using scan tool MB991502, read the diagnostic trouble code (DTC). CAUTION: To prevent damage to scan tool MB991502, always turn the ignition switch is to the “LOCK” (OFF) position before connecting or disconnecting scan tool MB991502. 1. 2. 3. 4. Connect scan tool MB991502 to the data link connector. Turn the ignition switch to the “ON” position.