My Nissan Xterra's temperature gage on the dash indicates that the vehicle is overheating only when driving and the Cool air or Air Conditioning is turned on. My thermostat is fine, and so are the radiator hoses, water pump and belts. Could it be ther temperature sending unit?
Could be the coolant temperature sensor or a bad thermostat-or an AC idle up solenoid not coming on to compensate the load of the compressor.
Have it block and pressure tested-do the block first to pinpoint a combustion leaking into the cooling system or a gas analyzer to sniff for hydrocarbons at the radiator fill neck.
Pressure test: do not do a pressure test if there's leakage at the headgasket this might cause coolant into the cylinders and lock it up or bend a connecting rod if cranked thereafter. The headgasket should be repaired before doing the pressure test.
If the block and pressure test passes check the following: Thermostat, Clogged radiator, Fan clutch, Radiator electrical fan, collaspe hoses, water pump.