* The AC condenser is located in front of the radiator. When the AC is on, the condenser draws air from the outside and passes it over the radiator. This can cause the temperature of the radiator fluid to rise, which in turn can cause the temp gauge to go up.
* The AC compressor puts a load on the engine. When the AC compressor is engaged, it puts a load on the engine, which can cause the engine to run hotter. This can also cause the temp gauge to go up.
* The AC system may be low on refrigerant. If the AC system is low on refrigerant, it will not be able to cool the air as effectively, which can cause the temperature of the radiator fluid to rise. This can also cause the temp gauge to go up.
If you are concerned about your temp gauge going up when your car is sitting with the AC on, you should have your car inspected by a qualified mechanic.
