In article , ndccpf1 wrote
The high pressure switch is a normally closed switch which opens if there is excessively high pressure in the high side of the AC system. This function used to be handled by a pressure relief valve until environmental laws outlawed venting refrigerants.
You either have excessive pressure on the high side of the AC system or a failed high pressure switch.
You'd best make certain (check the high side pressure) which it is before you add more refrigerant as some as suggested.