Code P0128: Coolant temperature below thermostat temperature control - Garage with CarParts.com (2023)

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Codes that illuminate the MIL (Check Engine) light with no apparent symptoms are ignored by many drivers. The problem with this strategy is that unless you have an active misfire (flashes if a misfire is bad enough), the MIL light only knows how to tell you there's a problem.does not say how many problems the ECM/PCM has detected.

So if you stop by the parts store, for example, or use your own scanner and get a P0128 code, say, and think, "I'll take care of that later," when that "later" finally comes around, you'll find that you're about have five or six different issues piled up behind the first one that lit the MIL.

What does the P0128 code mean?

Code P0128 means engine coolant temperature below the thermostat setting temperature. This means that the ECM/PCM has "detected" that the engine coolant temperature is consistently below the thermostat setting. In other words, thethe engine runs cooler than it should.

Code P0128 is set when the engine control module (ECM) detects that the coolant temperature is lower than the original equipment thermostat opening temperature. That means the engine is running too cold or the temperature is not rising fast enough. The engine must reach operating temperature immediately before the vehicle can switch to closed-loop mode.

In this mode, when everything is warmed up, the ECM will begin using the oxygen sensor(s) for fuel control. It's about fuel consumption and emissions.

Like so many other components that we don't even think about (like the PCV valve for example), the ECM/PCM has the opening temperature of the engine cooling system's thermostat in its programming. It's important to note that some OEMs will store a P0125 code if the condition that triggered the P0128 becomes so severe that the engine is running even cooler.

Read on for more information on P0128. You can also jump quicklyPossible causes.

Do you have other codes in your scan tool? You can search ourOBD-Codeliste.

A closer look at the P0128 code

A healthy engine should workover 200 degrees🇧🇷 The radiator fan on some vehicles will not even turn on until the coolant is reachedalmost 230 degrees🇧🇷 So if the engine is expecting 200-230°F (most thermostats open at just under 200°F) but the ECM only sees 170°F, you can see why the ECM/PCM algorithm will store the code and light up would mil

In most cases, if the engine temperature is lower than expected, the ECM will note and store this information. If this happens again, the ECM will register it, but will also store a P0128 code and activate the MIL light.

For this reason, P0128 is often referred to as a"Two trips" error🇧🇷 If the engine reaches operating temperature on two consecutive trips after the code has been set, the ECM/PCM will clear the code and clear the MIL.

The cold engine condition that will trigger a P0128can i or can i notsuffer damage🇧🇷 In addition to the obvious effects a cold running engine can have on emissions and fuel economy, an engine running too cold for too long can develop, for examplecrankcase sludgeEven if the oil is changed regularly.

One symptom you may or may not notice is the coolant temp gauge is colder and the heater doesn't work as well or at all on cold days - you just get cold air. A faulty heater is a huge motivator when it comes to taking care of a P0128.

What are the possible causes of the P0128 code?

It is important to understand the possible causes of a P0128 code so that you can take the necessary preventive measures.

The most common causes of P0128 are:

• Low coolant level(an air pocket trapped in the engine coolant sensor)
• The engine coolant thermostat is stuck in the open position or opens too early. If the thermostat is fully open instead of just open, a P0125 will be stored.
• The motorTemperature cooler sensoror one of the connected lines is defective – this is very rare.
• Defective intake air temperature (IAT) sensor - even rarer
• Cooling fan stuck(Many vehicles switch off the fan above a certain vehicle speed).
• ECM issues (e.g. software that needs updating) - very, very rare

What are the most common symptoms of the P0128 code?

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

• Check if the engine light comes on
• It takes longer to heat up
• When driving on the freeway, the engine temperature drops significantly
• temperature displaymeans colder than usual
• The heating does not work optimally
• Reduced fuel economy (generally not as noticeable with a P0128)
• Failed emissions testor monitors "not ready" for emissions testing

How to diagnose the P0128 code

There are numerous possible causes of the P0128 code. Therefore, diagnosis can be difficult. To get an idea of ​​how to fix the code, watch the video below:

How to fix the P0128 code

There are several reasons why the P0128 code can be stored. Therefore, there is no "magic" solution to the problem. You must accurately diagnose the code as detailed above and make any necessary repairs.

Note from ASE Certified Mechanic Richard McCuistian:

„Personally, I always start with “cheap and easy”. If the thermostat is easy to replace and doesn't cost much (which isn't the case on all vehicles) I always start there and then clear the code. If the thermostat doesn't care (i.e. engine still runs cold and P0128 returns) I've eliminated it as the cause.

The code can be triggered by anything from a low coolant level to a malfunctionYou may need to replace the thermostat, then you have to do your homework.

Important Announcement:If you decide to replace the thermostat yourself, you need to know that topping up the cooling system is not like pouring water into a bucket. Air tends to get trapped in some systems, so make sure you know how to properly top up the cooling system before you even begin replacing the thermostat. Ignoring this very important warning can lead to more than one type of disaster, including personal injury.

Also remember that all vehicles are different. When troubleshooting and repairing diagnostic trouble codes, you should refer to the factory repair information for your application.

Repair instructions like those from Chilton are helpful, and an ALLDATA subscription is even better. ALLDATA offers single vehicle subscriptions for DIYers with detailed factory repair information.

How Long Can You Drive With a P0128 Code?

While it is possible to drive short distances with the P0128 code set, you should have the problem dealt with immediately by a professional. This will help keep your vehicle running at its best and help prevent further damage.

More notes on P0128

If the coolant does not reach the ideal temperature, it can affect engine performance and fuel economy. Since this usually indicates a problem with the thermostat, whether you drive a Dodge, Ford, or Jeep, the P0128 code will likely show up when the coolant thermostat or temperature sensor is failing.

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

If the code appears, take your vehicle to a trusted mechanic and have it checked. Otherwise, the damage could spread to other parts of the vehicle, leaving you with more repairs and higher costs in the long run.

Prevent this from happening by identifying the symptoms of the p0128 code sooner and fixing what needs fixing.

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