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P0128 Fault Code

You can find information about the P0128 fault code on this page.

It is one of the most popular fault codes that people search for on this website (in the top 50).

We have given it a seriousness of 4/10 with 1 being the least serious and 10 being the worst.

P0128 Short Description

"Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)"

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Description Icon Long Description

Fault code P0128 is logged when the engine control module (ECM) detects that the engine coolant is not warming up at the desired rate. This can be that the coolant is taking too long to reach operating temperatures or that it does not reach operating temperatures at all.

The cooling system in an engine is designed to operate at a specific temperature, usually around 90 °C. In ideal circumstances, the coolant should reach this temperature within a few minutes of operation.

This can cause several issues like decreased fuel economy and higher idling revolutions per minute.

Another similar code is P0125, which represents the ECM not detecting enough heat in the engine.

Conversely, code P0217 indicates that the engine is overheating. However, overheating is a much more severe issue than low or slow-rising coolant temperature.

This fault can affect any brand of car, but some of the most common offenders are Ford, Mercedes-Benz, and several General Motors brands like Dodge, Jeep, and GMC.

Warnings Icon Other Signs of This Problem

For most cars, the ideal operating temperature can be seen as the middle point in your temperature gauge. This should ideally happen within a few minutes of starting and driving the car.

If this issue is occurring, the needle will likely be indicated below this point perpetually or for longer than the usual time.

In some cars, the needle does not reach this midpoint even when operating perfectly normally. Be sure to take note of where the needle reaches usually so that there is something to compare to if you suspect this issue.

Another symptom of this error is the heater will not get as hot as usual as there is not enough energy transfer between the engine and the coolant.

The Check Engine light will also display usually – followed by the P0128 fault code reported through an ODB2 reader.

Broken Down Icon What Could Be The Problem?

Due to the specific nature of this error, most causes will be limited to the cooling system alone.

A key area of concern is the thermostat. If the thermostat is damaged or stuck open, it will not be able to regulate the coolant pressure properly, preventing adequate heat exchange. The thermostat may also be opening too early.

Another fault could be with the coolant temperature sensor. If it is giving a false reading that is below the requirements, it can give off the fault code.

Fixed Icon Fixes

(You can view this YouTube video for a visual explanation of the problem along with 2 DIY methods to fix the P0128 fault)

If there is an issue with the thermostat or the thermostat housing, these can be replaced to fix the issue. This is typically a relatively cheap fix.

A thermostat can also be tested to check that is opening at the correct temperature through various means.

If there is no issue with the thermostat, another fix could be to replace a faulty coolant temperature sensor. Also, be sure to check the wiring and connection terminals for damage. If these are damaged, replace them.

Also, be sure to check that there is the right amount of coolant in the system. This is best done when the engine is cold.

Danger Icon Seriousness

Unlike an overheating engine, which is a severe issue, this P0128 fault code is relatively low in terms of seriousness.

If there are no serious symptoms like dramatically decreased fuel economy, there is no rush to fix the issue. However, it should be rectified within a few weeks ideally as prolonged operation in this state has the potential to cause damage.

P0128 Image

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