Test 1: Coolant Temperature Gauge Always Indicates Cold

1. Turn ignition off. Disconnect engine coolant temperature sender harness connector. Using a fused jumper wire, ground terminal "A" (Dark Green wire) at engine coolant temperature sender harness connector. Turn ignition on. If coolant temperature gauge indicates hot, go to next step. If coolant temperature gauge does not indicate hot, go to step  3.
2. Replace engine coolant temperature sender.
3. Turn ignition off. Disconnect instrument cluster harness connector. Check continuity of Dark Green wire between terminal "A" at engine coolant temperature sender harness connector and terminal No. 13 at instrument cluster harness connector. See Fig 1. If continuity exists, go to step  5. If continuity does not exist, go to next step.
4. Repair open in Dark Green wire between engine coolant temperature sender and instrument cluster.
5. Replace instrument cluster.

Fig 1: Identifying Instrument Cluster Harness Connector Terminals

Courtesy of GENERAL MOTORS CORP.