Check the fuses if the coolant temperature gauge isn't working. If the gauge reads hot soon after starting a cold engine, disconnect the wire at the coolant temperature sending unit. If the gauge drops, replace the unit. If it remains high, there could be a shorted wire or faulty gauge. If the gauge doesn't indicate after the engine warms up, connect the wire at the sending unit to a clean engine ground. If the gauge reads hot, replace the unit. If it doesn't work, there might be an open circuit or faulty gauge. To replace the unit, release
Radiator pressure, disconnect the wiring harness, apply sealant to the new unit's threads, and quickly install it. Tighten it, reconnect the wiring, refill the cooling system, and check for leaks and gauge operation.