* Idle Air Control (IAC) Valve: This is the most likely culprit. The IAC valve regulates the amount of air entering the engine at idle. A dirty, sticking, or failing IAC valve can cause erratic idle, especially when the engine is under load (like when stopped at a stop sign with accessories running). The surging up and down is classic IAC behavior.
* Throttle Position Sensor (TPS): The TPS tells the computer the throttle's position. A faulty TPS can provide inaccurate readings, leading to poor idle control and a check engine light. Similar to the IAC, a bad TPS can cause erratic idle, particularly at stops.
* Mass Airflow Sensor (MAF) Sensor: The MAF sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can cause inaccurate fuel mixture calculations, resulting in rough idle and potentially a check engine light. However, MAF issues often cause problems at higher RPMs as well, so this is less likely than the IAC or TPS.
* Vacuum Leaks: A leak in the vacuum system can disrupt the proper functioning of various engine components, including the idle system. This could manifest as erratic idle, especially at stops.
* Engine Coolant Temperature (ECT) Sensor: While less likely to cause the specific surging, a faulty ECT sensor can provide incorrect readings to the engine computer, leading to improper fuel mixture and possibly idle issues. A bad ECT sensor would usually trigger a code and possibly cause other problems.
What to do:
1. Get the Diagnostic Trouble Codes (DTCs): The check engine light is there for a reason. You need to get the DTCs read using an OBD-II scanner. Auto parts stores often offer free code reading services. This will give you a much more precise idea of what the computer has detected.
2. Check the IAC Valve: If you're mechanically inclined, you can try cleaning the IAC valve (plenty of YouTube tutorials show how). It's relatively easy to access and clean. However, if cleaning doesn't solve the issue, replacement is often necessary.
3. Inspect for Vacuum Leaks: Carefully inspect all vacuum hoses and connections for cracks, leaks, or loose fittings.
4. Consider professional help: If you're not comfortable working on your car yourself, take it to a qualified mechanic. They can diagnose the problem accurately and make the necessary repairs.
Important Note: Ignoring the check engine light can lead to further damage. It's best to address the underlying issue as soon as possible. The IAC valve is a strong suspect based on your description, but a proper diagnosis with a code reader is essential.
