2003 impala is it a temp sensor or cooling fan switch

I think first of all you need to verify that it is really overheating and isn't a sensor that has become faulty and is sending false information to the management computer that these days will drive the cooling fan, the temp gauge and any alarms.
An obd scanner should show a similar coolant temp and air temp when the engine is cold and then register a fairly smooth increase after starting until it drops when the thermostat opens and begins to climb again until working temperature is reached and the cooling fan begins to cycle (if electric).

A failure to register and then suddenly jump could be a faulty sensor but could also be indicative of a blocked degassing system or an excessive amount of gas in the cooling system.

It is important to observe the engine during warmup and keep monitoring the heat of various hoses, the thermostat operation and so forth.

check the code what is it then try fix.

If the check engine light is not on, I would want to check the computer with a scanner to see what the temp sensor is telling the computer the coolant temp is. The problem could be in the computer or the fan relay.

Would turn the key on no need to start it and unplug the ect engine coolant temp sensor. If fans come on then suspect a faulty etc which may not turn on the engine light. Which engine is in the vehicle so I can help locate the ect which is usually by the thermostat housing.

Thanks for choosing FixYa and welcome. Do you have a check engine light on? If it was ran a lot during the overheating, there may be engine damage. You may have a bad head gasket.

My guess is trapped air in the cooling system. If the cooling system was just filled with antifreeze without bleeding the air, it could cause sporadic overheating. It could also be the cooling fan not coming on at high speeds. Next time the temperature rises, turn on the defroster for th front windshield. That should cause the air conditioner to come on, which in turn will turn on the cooling fan. If the temperature drops, I would suspect the cooling fan sensor is faulty.

dose it run hot with ac on or open hiway?

coolant temp sensor defective. look through manual for location of this sensor.

replace thermostat, this fixed my car, cooling fans keep coolant below temp guage parameters.

The electric fans on some vehicles dont stay on all the time. If the radiator is not burningyou then i would suspect the guage is bad. The engine would throw a code if the fan was not working properly, so im pretty sure that your fan is working properly. I hope that helps you out.