P0128 - Coolant Thermostat (Coolant Temp Below Regulating Temp)

The Technical Definition: The engine is not reaching its optimal operating temperature within the expected amount of time.

Common Symptoms: Check engine light is on. The temperature gauge reads lower than the normal halfway mark. The heater might not blow fully hot air.

Typical Fix: The thermostat is stuck open. Replacing the thermostat housing assembly and topping off the coolant with the correct HOAT or OAT Mopar fluid almost always resolves this.