A short in the sending unit, its wire or a faulty gauge.
Unplug the sending unit wire and turn ignition switch on, does the gauge needle max out?

Answered by KHLow2008 (expert)
39,806 answers provided

Thanks for the answer. Have already unpluged the unit wire and the guage comes down to zero when the car is turned on. Any other suggestions.

Response from du toit
2 questions asked

Either the sending unit is or gauge is bad. If the original and replacement sensor is showing same symptoms, the gauge is more likely to be the cause.

TEMPERATURE GAUGE & SENDER

Wiring Harness Operational Test

1. Unplug connector at coolant temperature sender. Turn ignition on. Temperature
gauge should indicate COOL. Turn ignition off. Connect a 12-volt, 3.4-watt test light between coolant temperature sender harness connector terminal and ground.

2. Turn ignition on. Test light should glow and temperature gauge should slowly
move to HOT. If gauge functions as described, replace sending unit. If gauge
does not function as described, perform TEMPERATURE GAUGE RESISTANCE TEST below.

Temperature Gauge Resistance Test

Remove instrument cluster. Using ohmmeter, check gauge resistance across appropriate terminals. See Fig. 17. Ensure ignition is off and harness
connector is unplugged from instrument cluster. See TEMPERATURE GAUGE RESISTANCE SPECIFICATIONS table. Replace gauge if not within specification.
If gauge is within specification, repair open or short circuit in wiring harness.

Answered by KHLow2008 (expert)
39,806 answers provided