[Richard]

GL is low inside fan
GH is high inside fan
Y1 is compressor 1
Y2 is compressor 2, which you don’t have
Red is 12 VDC +
Black is ground
White is heat, signal to AH

With that info, use your voltmeter to see if Tstat is doing what it should
Remember there is at least a two minute or longer delay when making a change on that particular Tstat before the change is enacted. That is to protect the compressor from restarting against high head pressure.

If that works, then you must access control board before determining anything else.

I will help you, but my motto is “In God we trust, everybody else bring data” meaning that I will ask you very specific questions in helping you diagnose the unit. The first one is how cold is the exit and return air? Cool, room temp, hot are not the right answers. The temperatures measured with a digital thermo are the right answers.

Second question, how many amps is the unit pulling with the temp set point at least three degrees below ambient. This would be best measured at the power feed into the unit, second best at the breaker box, and third best looking at your overhead ammeter.