* Idle Air Control (IAC) Valve: This valve regulates the amount of air entering the engine at idle. A malfunctioning IAC valve can cause the engine to stall, especially after a short idle period when the engine is already running lean. It might be dirty, sticking, or failing internally.
* Throttle Position Sensor (TPS): This sensor tells the computer the position of the throttle. A faulty TPS can provide incorrect information, leading to poor fuel mixture and stalling.
* Mass Airflow Sensor (MAF) Sensor: This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can provide inaccurate readings, leading to a lean fuel mixture and stalling.
* Crankshaft Position Sensor (CKP) Sensor: This sensor tells the computer the position of the crankshaft, crucial for ignition timing. A failing CKP can cause the engine to cut out intermittently.
* Fuel Pump or Fuel Filter: A weak fuel pump or a clogged fuel filter can restrict fuel delivery, especially when the engine is idling and demands are lower. The engine might be getting enough fuel under load but not at idle.
* Vacuum Leaks: Leaks in vacuum lines can disrupt the proper functioning of various engine components, leading to stalling.
* Battery/Alternator: While less likely to cause an immediate stall *only* at idle, a weak battery or failing alternator could lead to insufficient power to the engine's control systems, causing it to shut down.
* Computer Problems (PCM/ECM): In some cases, a faulty engine control module (PCM or ECM) could be the root cause. This is less common but should be considered if other components check out.
* Spark Plugs/Ignition Coils: While less likely to cause a stall *only* at idle, worn spark plugs or failing ignition coils can cause misfires, leading to rough running and potential stalling.
What to do:
1. Check Engine Light: Is the Check Engine Light on? If so, get the diagnostic trouble codes (DTCs) read using an OBD-II scanner. This will give you a starting point for diagnosis.
2. Visual Inspection: Look for any obvious vacuum leaks, loose connections, or damaged wiring.
3. Professional Diagnosis: If you're not comfortable performing your own repairs, take your Ford Focus to a qualified mechanic. They have the tools and expertise to properly diagnose the problem and perform the necessary repairs.
Don't attempt complex repairs yourself unless you have the necessary knowledge and tools. A misdiagnosis can lead to further damage and costly repairs. Start with the simple checks (engine light, visual inspection) and proceed from there.
