I've noticed recently in my 2002 WRX that when I press the A/C button, the compressor sometimes does not come on at all. Other times it comes on, but only for about 10 seconds, then shuts off. I thought this was an issue so I took it to a shop this morning. They just called saying this is normal operation and that the compressor shuts off when it hits its intended pressure. I appreciate them not ripping me off and making unnecessary repairs, but it seems that something is wrong if the compressor doesn't even kick on at all sometimes. I remember in past winters using A/C on "hot" quite a bit to help de-fog the interior windows, so they don't steam up. This doesn't seem right that I can't have the A/C running now. Thoughts?

 

It's normal, your A/C system operates on pressures between what they called a high side and low side in the A/C system. Without going technical, It is normal for your A/C to cycle to prevent the seals from drying out and extend the life of your A/C compressor. There's no computer, is all operated by the HVAC control switch (climate control on your dash), pressure switches and temperature sensors within the HVAC system; pressures mostly control whether or not the A/C turns on or off. The A/C will shut off during, hard engine load (i.e racing, turning the steering wheel lock to lock). Also, the A/C will not work if there's no pressure in the system (i.e. a leak), this is to prevent the A/C compressor burning out.

It is recommended during winter season to run your A/C system for at least 5 min once or twice a month. However, if you neglect to do it, it does it for you by turning on automatically when you have the heater defroster on. This is to improve optimal performance of your A/C compressor all year round.