Q: What would cause the temperature gauge in a 1993 geo prism to work fine when idling but when driving it drops to cold ?

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The temperature gauge works fine when idling but when driving it drops to cold and also when idling the heat works fine but when driving it also drops to like a luke warm

My car has 238516 miles.
My car has an automatic transmission.

Assuming that the temperature gauge and sending unit are in good shape, what may be happening is the engine cooling thermostat may be sticking in the open position causing engine temperature to fall below normal once you are at speed. If the thermostat has been in the car for, say, 100,000 miles I would first consider replacing the thermostat. If you request that service, a certified Mechanic, dispatched by YourMechanic right to your location, will replace it for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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