If your vehicle is showing temperature gauge fluctuates erratically while driving, here are the most likely causes and the parts you'll need to fix each one.
Air bubbles passing by the temperature sensor cause the gauge to read erratically, as air transfers heat differently than liquid coolant. This is often caused by a slow leak or failing to properly bleed the system after a recent coolant flush.
A failing sensor or a loose electrical connection can cause the gauge needle to instantly spike or drop. Unlike actual overheating, these electrical fluctuations happen faster than a liquid can physically change temperature.
If the thermostat sticks open and closed erratically, it causes sudden rushes of cold coolant into the engine, making the gauge sweep up and down. This can also happen if a new thermostat was installed backwards.
A water pump with a loose internal impeller or a slipping drive belt can cause intermittent coolant circulation. When circulation stops, the temperature spikes; when it catches again, the temperature drops.
Combustion gases leaking into the cooling system create superheated air pockets that cause the gauge to bounce erratically. This is a severe failure usually accompanied by white exhaust smoke or milky engine oil.