Temperature gauge is fluctuating while driving.

An accurate temperature gauge will always fluctuate because the temperature in the engine fluctuates.

Usually the only time this is really noticeable is when the engine has been working hard and suddenly has to wait at a junction or similar and the gauge can rise considerably.

You didn't mention by how much the gauge fluctuates?
As long as it remains approximately in the normal zone it is acceptable but if it climbs into the hot zone or falls into the cold zone it is not acceptable and the cause should be investigated as soon as possible.

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These details will help.
Thanks.

Watch the temp gauge when the electric radiator fans come on. The temp sensor that controls when the fan comes on may be faulty.

This can be caused by a bad controls or bad blend door motor, the door motor and controller will need to be tested to see which is causing the problem,

when you say temp fluctuates what happens? are you using the a/c and you turn temp down one click and it goes to hot ?
Is the temp going from hot to cold hot to cold as you drive?

There could be a number of causes of a fluctuating temperature gauge but most commonly is the thermostat. It could also be a head gasket, cooling fan problem or the actual temperature sending unit. I hope this helps!

First of all you have a bad heater core that needs to be replaced. Second you may have a bad temperature sensor. Make sure the radiator if full of water

Definitely check your fluid level in your radiator. Top it off all the way. I've seen that happen multiple times on cars that were low on anti-freeze. If that doesn't work try replacing your thermostat. It's cheap and the easiest place to start if topping off the fluid doesn't get you fixed up.

Here's the thing. The ECU will shut down AC if the engine temperature is too hot. If the coolant temperature sensor is malfunctioning (lying to the ECU) telling it that the temperature is hotter than it actually is, the computer will turn the AC OFF.

The shut off valve on the heater core was left shut during the repair on your coolant system. the repairman forgot to re open the shut off valve . this is the problem. open the shut off valve and, the air will become hot again. as for the gauge fluctuations, the actual gauge may be malfunctioning.

NO.... BUT THEY COULD HAVE COMPROMISED THE WIRING OR SWITCH AT OIL FILTER AREA