* Weak/Failing Alternator: This is the most common cause. The AC compressor demands significant power. A weak alternator can't keep up with the increased load, causing the voltage to drop. This voltage drop can disrupt the engine's computer, leading to a stall. Get your alternator tested for output voltage both at idle and with the AC on.
* Bad Battery: A weak or failing battery can't supply the extra current needed by the AC compressor. Similar to a weak alternator, this can cause voltage drop and stalling. Have your battery tested for its ability to hold a charge and its cranking amps.
* Dirty or Corroded Battery Terminals/Connections: Even a good battery can't supply power if the connections are poor. Clean and tighten all battery terminals and any other connections related to the charging system.
* AC Compressor Issues (Less Likely, but Possible): While less likely to cause an immediate stall, a seized or heavily worn AC compressor could put an exceptionally high load on the system, potentially contributing to the problem. However, this would usually manifest as significant noise or strain on the engine *before* stalling.
* Wiring Problems: A short circuit or faulty wiring in the AC system's electrical circuit could draw excessive current, overloading the system and causing a stall. This is less likely but possible.
* Engine Problems (Least Likely): While less probable, underlying engine issues could be exacerbated by the added load of the AC. This could include things like low compression or fuel delivery problems. This is less likely given the direct correlation to the AC being turned on.
Troubleshooting Steps:
1. Check Battery and Alternator: This is the first and most important step. Have a mechanic or auto parts store test both. This is the quickest and cheapest way to rule out the most common culprits.
2. Clean Battery Terminals: Clean and tighten the battery terminals and any connections related to the charging system. Corrosion can significantly increase resistance and lead to voltage drop.
3. Check AC Compressor (If other steps fail): Listen for unusual noises from the compressor when the AC is engaged. A seized compressor will make a grinding or squealing sound.
4. Professional Diagnosis: If the above steps don't solve the problem, take your Focus to a qualified mechanic for a proper diagnosis. They have the tools and expertise to identify more complex electrical or engine issues.
Don't continue driving the car with this problem, as it could lead to further damage or leave you stranded. Address the issue as soon as possible.
