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when a temp gauge drops to cold ,it indicates a problem in the cooling system
temp gauge sender units need water to activate the resistance that makes the gauge read so a falling gauge indicates no water around the sender unit --- air lock in the system--faulty wire connection to the gauge sender unit
the ac unit is not connected to the temp gauge so there may be another separate problem there
the temp sensor is an entirely different unit to the temp gauge sender unit and the short term working would be from an altered coolant level from changing that sensor --air lock in system
the temp sensor connects to the ECM to activate the fans and monitor engine operating temp so as to keep the air/fuel mixture exact for the engine conditions
the temp sender unit just makes the temp gauge read
a faulty thermostat will either be stuck open and the engine temp will take a long time to rise or it will be stuck closed and that will cause an overheat very quickly
it could very well be a bad gauge. when it shows the engine to be overheated or at the least very hot, check the actual temp at the water outlet where the upper radiator hose attaches to the engine. use a non contact thermometer to check the temp so as not to get burned in the process. if the thermometer reads way different than the gauge, the gauge is bad.
If you are not getting a signal to your gauge, your car can still run. You just won't know if your car is overheating until it is too late. A bad thermostat won't keep your gauge from working. The temperature sensor that sends the signal to the gauge will keep it from working.
The thermostat helps your car in extreme weather. It helps your engine warm up in extreme cold so your heater will work sooner. And it helps you car cool itself off in extreme hot weather. As your engine water temp rises, the thermostat will open allowing the water in the radiator (that has been cooling) to flow into the engine block. The water that was in the block will go into the radiator to begin cooling.
It could be 3 things: Temp sensor, thermostat, or fan relay. Fans don't usually turn on until car has been idleing for a while. I first would change thermostat, then relay, then temp sensor. Coolant must be 50/50 mixture for water pump to have proper lubricant from antifreezer. Do not go any further than a 60water/40antifreeze mix
Assuming the t-stat is good, I'd look at the wiring next. Make sure the connector is clipped onto the water temp sensor/back of the gauge and that the wiring is not chaffed, etc. If that all looks good, replace the coolant temp sensor...they're $20 or so. If it still does it, the gauge is bad.