After 4 years of use, the compressor runs constantly for this window A/C unit. Regardless of the temperature and thermostat (which seems to work) the compressor stays on and cold air continues to blow. When I turn the unit off the compressor continues to hum. I have to unplug the unit to shut it off. When I plug back in and try again, same problem. Two questions:

1) which part do i need to replace
2) is it possible to replace it myself

The unit is in a very tight spot and nearly impossible to remove from window entirely - so i'd like to fix if possible rather than replace - assuming i can access and replace the right part without removing entire thing from window.

thank you!!