Has CHT - cylinder head temp. sensor . This sensor is a input to the PCM - engine computer . The PCM sends engine temp info on serial data network to the instrument cluster . I/C has a microprocessor inside . Have it check for DTC'S - diagnostic trouble codes . Not OBD2 codes , B - body codes . The gauge could be the problem , which means replacing the instrument cluster .
Engine coolant temperature information is received by the instrument cluster from the PCM over the SCP network. Temperatures over 121°C (275°F) will display hot (H). If the engine coolant temperature data is missing or invalid for five seconds, the instrument cluster will move the engine coolant temperature gauge below the cold (C) position.
The best way to diagnose this is with a scan tool , factory or a professional type . Know how the system works and not replacing parts is the proper way to diagnose .
SCP (J1850) Invalid or Missing Data for Engine Coolant
CARRY OUT the PCM self-test.
- A single gauge does not return to zero with the ignition switch off
- INSTALL a new instrument cluster; REFER to Instrument Cluster . TEST the system for normal operation.
- Incorrect temperature gauge indication
- Instrument cluster.
- Engine coolant temperature sensor.
I should also point out that if the temp sensor were bad the vehicle may not run right .
Using the factory scan tool
Trigger instrument cluster active command ENGCOOLNT. Monitor the engine coolant temperature gauge while adjusting the ENGCOOLNT active command to read 50% and 100%.
- Does the temperature gauge start at cold, move to half at 50%, and full hot at 100%?
REFER to the Powertrain Control/Emissions Diagnosis (PC/ED) manual.
INSTALL a new instrument cluster; REFER to Instrument Cluster . REPAIR the circuit. CLEAR the DTCs. REPEAT the self-test.
Your best bet , take it to a qualified repair shop !