If the coolant temperature gauge isn't working, check the fuses. If it shows excessive temperature, consult the manual. If it reads HOT on a cold start, disconnect the sender wire. If the gauge drops, replace the unit. If it stays high, the wire or gauge could be faulty. If the gauge doesn't indicate after warming up, disconnect the wire at the unit and connect it to a clean engine ground. Also, reconnect the connector, refill the cooling, and check for leaks.