P0128 OBD-II Trouble Code: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Check Engine Light

P0128 code definition

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What the P0128 code means

The P0128 code means that the engine coolant is not getting warm enough, fast enough. In other words, there is a problem with the engine operating temperature not being reached within a certain amount of time. The Engine Control Module (ECM) determines this length of time based on ambient temperature.

What causes the P0128 code?

There are two main causes for P0128:

  1. The most common cause is an engine coolant thermostat that has stuck open or is opening prematurely.

  2. The next cause is a problem with the engine coolant temperature sensor or the wiring associated with the sensor.

What are the symptoms of the P0128 code?

  • Heater is not hot enough due to the coolant temperature in the engine being low

  • Car takes too long to warm up, or the temperature gauge does not go up as high as it used to

How does a mechanic diagnose the P0128 code?

Diagnostic methods can vary from mechanic to mechanic but the first thing I check is to see how hot the coolant temperature is when coolant starts flowing through the radiator hose connected to the thermostat.

Generally speaking, the hose should be barely warm until the thermostat opens. When the thermostat opens, very hot coolant should start to flow and quickly warm the radiator hose up. You have to be careful because it is hot enough that you could be burned.

If the radiator hose just slowly warms up and doesn’t get hot all of a sudden, the cause is the thermostat being stuck open. If the hose gets hot all of sudden like it should, then the next thing to check is the reading you are getting from the coolant temperature sensor.

To do this accurately, you need a scan tool, but a reading is also possible to obtain if your car a has a temperature gauge. You need to compare this reading to one obtained by an infrared thermometer. The temperature from the thermometer should be around 200 degrees fahrenheit, normally. If the scan tool reading does not match the thermometer reading closely, then there is an issue with the sensor side of the car.

If this is the case, the connectors will typically deteriorate and cause a poor connection. If the connector is okay, then generally the sensor is at fault.

In very rare cases, there can be a problem with the supply voltage that is supplied by the Engine Control Module. I won’t go into detail on this because you need factory-level information that varies car by car, and you need good knowledge of electronic diagnostics to prevent causing damage to the vehicle.

Common mistakes when diagnosing the P0128 code

  • Replacing the thermostat without checking the vehicle:

If you ask any mechanic about a P0128 code, most certainly at least 9 out of 10 of them will tell you the thermostat is at fault. The vast majority of the time, they would be right. However, every once in a while it is possible that the thermostat will not fix this code.

How serious is the P0128 code?

P0128 is not a very serious code. However, if not repaired and the vehicle is used over a long period of time, there is a small chance that water contamination of the oil could cause deterioration and engine damage. Since this code sets when the coolant isn’t getting hot enough, it means that the engine isn’t getting hot enough for the condensation in the engine to be burned off; that water could end up in the oil. Any resulting engine damage would take a period of years most likely, but the chance is there.

What repairs can fix the P0128 code?

Additional comments for consideration regarding the P0100 code

  • Typically, this code will only show up in the cold winter months.

During the warmer months, the ambient temps will help the vehicle warm up normally, whereas the cold weather will make it harder for the engine to warm up.

Need help with a P0128 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote or book and appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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