* Low Refrigerant: A dead battery doesn't directly cause low refrigerant, but the prolonged inactivity might have allowed for a leak to develop that previously went unnoticed. This is the most common reason for AC failure. You'll need to have a mechanic check the refrigerant level and for leaks. Simply adding refrigerant without finding and fixing a leak is a waste of money.
* Blown Fuse: Check the fuse box (usually under the hood and/or inside the dashboard) for any blown fuses related to the AC system. Consult your owner's manual to locate the relevant fuses.
* AC Clutch Relay: The AC compressor is engaged via an electromagnetic clutch. A bad relay can prevent the compressor from turning on, even if the refrigerant is fine. This is relatively inexpensive to replace if it's the culprit.
* Compressor Issues: The compressor itself could have failed (though less likely directly due to a dead battery). A mechanic can diagnose this.
* Electrical System Issues: The large voltage drop from a dead battery can sometimes damage sensitive electronics. While not common with just the AC, it's a possibility. A mechanic might need to check for other electrical issues.
What to do:
1. Check the obvious: Make sure the AC is turned on (both the fan and the AC button). Ensure the temperature is set low enough.
2. Check the fuses: As mentioned above.
3. Listen for the compressor: When you turn on the AC, you should hear a faint clicking sound (the AC clutch engaging) followed by a whirring sound from the compressor under the hood. If you hear neither, there's likely an electrical problem.
4. Have it professionally diagnosed: If the above steps don't resolve the issue, a professional mechanic specializing in automotive AC systems is the best course of action. They have the tools to check refrigerant levels, diagnose electrical problems, and determine if the compressor or other components need repair or replacement.
In short, there's no "reset button" for the AC. The lack of cooling is almost certainly due to a mechanical or electrical problem, often unrelated to the dead battery beyond perhaps revealing a pre-existing issue.
