I drive a 2013 Chevy Sonic.
Basically, the air conditioning does not blow cold air if the car is idling, only when in motion. It seems to blow the coldest during long periods of driving fast, such as on the freeway. Once I have slowed down significantly or stopped, the air blowing out gets warmer and warmer. If I leave the AC on cold long enough (more than about five minutes) while staying slow enough (idling or driving >10mph during heavy traffic), then the engine overheating warning appears on my dashboard:
When this happens, I stop, open the windows, and turn on the heat to max. This typically makes the warning go away quickly, within 20-30 seconds. The warning will not come back unless I return to idling with the cold AC on. This has happened at least half a dozen times in the past year, so I typically just turn the AC off when going slow to avoid it.
The first time it overheated I refilled the coolant, but that didn't end up having any noticable effect.
Is there anything I can check on to try to figure out the source of this problem? Is it something urgent that I should get checked out right away?