What is P0128?
Engine code P0128 means “Thermostat Rationality/Engine Coolant Temperature Below Thermostat Regulating Temperature.” It shows up when the coolant temp sensor detects a coolant temp below the predicted engine coolant temperature.
Since P0128 is a generic engine code, it indicates coolant temp issues in all OBD-II equipped cars manufactured after 1996. But specific repairs may depend on the car’s make and model.
Your engine coolant should reach a defined thermostat regulating temperature within a specific amount of time after you start the engine. If it doesn’t, the Engine Control Module (ECM) or Powertrain Control Module (PCM) generates the P0128 error code.
Your car must reach the correct engine operating temperature so that the fuel system can enter a closed-loop and oxygen sensors can ensure proper air to fuel ratio for better car performance. Ideally, the operating coolant temperature should be around 160°F (71°C) within 15 minutes of starting your car. If the coolant temperature takes more time to reach this value, or if the temp gauge reads abnormal engine temperature levels, it would cause a P0128 engine code.