There are some things to look at; but it can be simple to make a system 'not work'. But without taking parts of the car off OR putting gauges on the system, there is NO WAY to know what's going on with the AC system (or ANY AC system, house included).
- Can look at the caps on the High/Low sides. The Schrader valves do leak and you need the caps. If they loosened the caps, that could cause a slow leak (gas gets trapped in the cap. loosening it allows Freon to escape).
- They could also have taken the caps off and pushed the valve. Release a bunch of Freon. Now you are low. Look for oil around the caps. Usually venting out in large quantities rapidly leaves the refrigerant oil where they vent.
- Look at the system (most parts are bare aluminum). Is there oil leaking somewhere?
- Are the sensors plugged in all the way? Notice anything hanging or not plugged in? Taking off the low pressure switch would prevent the system from coming on. Should have noticed that pretty quickly, but that's a quick way to disable the AC system to get you to come back. Takes 10 seconds to fix, but I can charge you $1,000 for a new system.
Short answer: yes, technically.
Longer answer: assuming they are sloppy, you might get some clues. If they are good, not really. But I would be HIGHLY suspicious of anything that starts acting funny, especially a couple of days later, after someone started mentioning it to me. And it was working fine beforehand.
~egon
