Definition of the P0128 code
Cooling thermostat (coolant temperature below thermostat setting temperature)
What code P0128 means
The P0128 code means that the engine coolant is not heating up fast enough. In other words, there is a problem in that the engine’s operating temperature is not reached within a certain time. The engine control module (ECM) determines this time based on the ambient temperature.
What causes code P0128?
There are two main causes for P0128:
- The most common cause is an engine cooling thermostat that gets stuck or opens prematurely.
- Another cause is a problem with the engine coolant temperature sensor or its associated wiring.
What are the symptoms of the P0128 code?
- The heater is not hot enough due to low engine coolant temperature.
- The car takes too long to warm up or the temperature gauge does not rise as high as before.
How can a mechanic diagnose the P0128 code?
Diagnostic methods may vary from mechanic to mechanic, but the first thing I check is the coolant temperature when the fluid starts coming out of the radiator hose connected to the thermostat.
In general, the hose should barely be hot until the thermostat opens. When the thermostat opens, the very hot coolant should start to flow and quickly heat the radiator hose. Care should be taken as it is hot enough to get burned.
If the radiator hose heats up slowly and does not heat up quickly, the cause may be a blocked thermostat. If the hose suddenly heats up as it should, the next thing to check is the coolant temperature sensor reading.
To do this accurately, you need a scan tool, but you can also get a reading if your car has a temperature gauge. Compare this reading to the reading obtained with an infrared thermometer. The temperature of the thermometer should normally be around 200 degrees Fahrenheit. If the scan tool reading does not agree with the thermometer reading, then there is a problem with the sensor on the side of the car.
If this is the case, the connectors usually fail and cause a bad connection. If the connector is OK, the sensor is usually the culprit.
In very rare cases, there may be a problem with the supply voltage, which is supplied by the motor control module. I won’t go into detail on this, as it requires factory information that varies from car to car, and a good knowledge of electronic diagnostics is required to avoid damaging the vehicle.
The most common errors when diagnosing the P0128 code.
- Replace the thermostat without checking the vehicle:
If you ask any mechanic about the P0128 code, at least 9 out of 10 will most likely tell you that the thermostat is the culprit. In the vast majority of cases, they would be right. However, in some cases, the thermostat may not clear this code.
How serious is the code P0128?
P0128 is not a very serious code. However, if it is not repaired and the vehicle is used for a long time, there is a small possibility that oil contamination with water could cause the engine to deteriorate and become damaged. Since this code is set when the coolant is not hot enough, it means that the engine is not hot enough for condensation in the engine to burn off; water can get into the oil. Any resulting engine damage would probably take years, but the possibility exists.
What fixes can repair a P0128 code?
- Replace the thermostat (the most common repair).
- Replace the coolant temperature sensor or the wiring and connector associated with the sensor that is defective.
Additional notes on P0100 code
- Typically, this code only appears during the cold winter months.
- In the warmer months, the ambient temperature will help the vehicle warm up normally, while cold weather will make it more difficult for the engine to warm up.
