Error P0128 – how to fix?

Code definition P0128

Coolant thermostat (coolant temperature below thermostat set temperature)

What does the code P0128 mean

The code P0128 means that the engine coolant does not warm up quickly enough. In other words, there is a problem that the engine operating temperature is not reached within a certain time. The engine control module (ECM) determines this time based on the ambient temperature.

What causes the code P0128?

There are two main reasons for P0128:

• The most common cause is the engine coolant thermostat stuck or opens prematurely.
• Another cause is a problem with the engine coolant temperature sensor or the wiring associated with it.

What are the symptoms of the P0128 code?

• The heater is not warm enough because the engine coolant temperature is low.
• The car takes too long to warm up or the temperature indicator 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 fluid begins to flow from the radiator hose connected to the thermostat.

In general, the hose should be barely warm until the thermostat opens. When the thermostat opens, very hot coolant should begin to flow and heat the radiator hose quickly. Be careful as it is hot enough to burn you.

If the radiator hose heats up slowly and does not heat up suddenly, 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.

You need a scan tool to do this exactly, but it’s also possible to get a reading if your car has a temperature gauge. Compare this reading with the reading obtained with an infrared thermometer. The thermometer temperature should normally be around 200 degrees Fahrenheit. If the scan tool reading does not match the thermometer reading, then there is a problem with the car side sensor.

If this is the case, connectors usually fail and cause a bad connection. If the connector is OK then usually the sensor is the culprit.

In very rare cases, there may be a problem with the supply voltage that is supplied by the engine control module. I will not go into detail on this as it requires factory level information which varies from car to car and good knowledge of electronic diagnostics is needed to avoid damaging the vehicle.

The most common errors when diagnosing the P0128 code

• Replacing the thermostat without checking the vehicle:

If you ask any mechanic for 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 P0128 code?

P0128 is not a very serious code. However, if it is left unrepaired and the vehicle is used for an extended period of time, water contamination of the oil is unlikely to result in engine deterioration and engine damage. As this code is set when the coolant is not hot enough, it means the engine is not hot enough for condensation in the engine to burn; the water may end up in the oil. Any resulting engine damage would probably last for years, but the possibility exists.

What repairs can fix the code P0128?

1. Replace the thermostat (most common repair).
2. Replace the coolant temperature sensor or the wiring and connector associated with the sensor that is damaged.

Additional notes for the P0100 code

• Usually, this code only appears in the cold winter months.
• In warmer months, the ambient temperature will help the vehicle warm up normally, while cold weather will make it difficult to warm up the engine.