OBD-II Trouble Codes

P0128 Code: Coolant Temperature Below Thermostat Regulating Temperature

Reading Time: 3 minutes

When the check engine light pops on in your car, you immediately start to wonder what it means, and whether it’s something serious.

Retrieving diagnostic trouble codes (DTCs) can help give you an idea. You see, anytime the check engine light comes on, your car’s computer also stores a corresponding code in its memory—and that code could be a P0128.

check engine light illuminating possibly because of code P0128
Code P0128 may trigger if the engine control module (ECM) determines that coolant temperature is lower than desired or does not increase quickly enough.

What Does Code P0128 Mean?

Code P0128 stands for Engine Coolant Temperature Below Thermostat Regulating Temperature. The code is set when the engine control module (ECM) determines that coolant temperature is lower than desired or does not increase quickly enough. The engine must reach operating temperature promptly so that the vehicle can enter “closed-loop” mode.

In this mode, when everything is warmed up, the ECM references the oxygen sensors for fuel control, resulting in improved fuel economy and reduced emissions.

In most cases, if the engine temperature is lower than predicted, the ECM notes that information and keeps it on record. When it happens again, the ECM notes it but also activates the Check Engine Light and stores a P0128 code.

For this reason, P0128 is often known as a “two-trip” fault.

What are the Possible Causes of the P0128 Code?

Coolant temperature sensors incur damage after a while. It’s important to know the possible causes of a P0128 code so you can take the necessary preventive measures.

The most common causes of P0128 include:

What are the Common Symptoms of the P0128 Code?

Here are the symptoms you are most likely to experience if your vehicle is triggering the trouble code P0128:

How to Diagnose the P0128 Code

There are numerous potential causes for code P0128. As such, diagnosis can be difficult. For an idea of how to troubleshoot the code, check out the video below:

How to Fix the P0128 Code

There are multiple reasons why code P0128 might be stored. Therefore, there isn’t a “magic bullet” fix for the issue. You’ll need to diagnose the code accurately, as outlined above, then perform any necessary repairs.

The code could be triggered by anything from a low coolant level to a faulty thermostat that you may need to replace, so you must do your homework.

Also, keep in mind that all vehicles are different. When troubleshooting and repairing diagnostic trouble codes, you should consult the factory repair information for your application.

Repair manuals, such as those from Chilton, are useful, but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.

How Long Can You Drive with a P0128 Code?

Although it may be possible to drive short distances with code P0128 set, you should get the issue fixed by a professional right away. Doing so will help ensure your vehicle keeps running at its best while also helping to prevent any additional damage.

Other Notes About P0128

When the coolant does not reach the optimum temperature, it can cause poor engine performance and bad fuel economy. Since this often indicates a problem with the thermostat, regardless if you are driving a Dodge, a Ford, or a Jeep, the P0128 code is likely to appear once your coolant thermostat or temperature sensor fails.

Of course, as was mentioned earlier, there are other potential causes for the code, as well.

When the code pops up, bring your vehicle to your trusty mechanic and have it checked. Otherwise, the damage can progress to other parts of the vehicle, leaving you with more repairs and larger expenses to take care of in the long run.

Prevent this from happening by detecting the symptoms of code p0128 earlier on and addressing what needs fixing.

mechanic checking coolant reservoir for coolant level
You may also get Code P0128 if the engine coolant temperature sensor or one of the wirings connected to it has issues.
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