P0128-THERMOSTAT RATIONALITY
When Monitored: The engine running. During cold start.
Set Condition: The PCM predicts a coolant temperature value that it
actual coolant temperature. If the two coolant temperature values are
(18°F) of each other an error is detected. Two Trip Fault.
Something is either taking TOO long to warm up, or it's warming up too quickly. Possibly low fluid, fan problems, incorrect thermostat, broken thermostat, etc.