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The P0126 is a trouble code within a small family of similar codes that reflect an engine that is running too cold. Find out more about why this code sets and what signs you should look out for with this brief guide.

What Does the P0126 Code Mean?

Diagnostic trouble code (DTC) P0126 stands for “Insufficient Coolant Temperature for Stable Operation.” This code sets when the engine does not reach normal operating temperature within a specific time period.

When the engine reaches its specified operating temperature range, it will be able to function optimally. This allows the powertrain control module (PCM) to effectively control the fuel and ignition systems, as well as the emissions output.

The temperature range is typically between 195-220°F and must be attained within a specific amount of time. The specific values depend on the make of the vehicle. The P0126 code sets if the coolant temperature stays below the specified range.

car 4 cylinder engine running
P0126 code sets when the engine does not reach normal operating temperature within a specific time period.

Once this happens, the PCM cannot rely on the data it receives from the oxygen (O2) sensors in the exhaust system. The PCM may then rely on a limp-home type of fuel and ignition control program. This program raises the level of exhaust gas pollution and may trigger the check engine light. However, not all vehicles may have this same fail-safe function.

The P0126 code is common in Mazda vehicles, though other vehicles may also set this code. In fact, Mazda released a few technical service bulletins addressing this trouble code.

About Engine Operating Temperature

An engine’s operating temperature can affect its performance and emissions. If the engine runs too cold, not only fuel economy  but also engine mechanical health will be impacted. Again, fuel economy will be reduced, but secondly, sludge can form in the crankcase on an engine that runs too cold for too long. “Mom and pop” vehicles that are driven short distances and never allowed to warm up completely suffer a similar fate.

But mostly, the P0125, P0126, and P0128 codes are structured for emissions control. A healthy engine needs to run at least 200 degrees. Some vehicles don’t even turn on their cooling fans until the coolant reaches nearly 230 degrees. Take note that codes like P0126 must be addressed as soon as possible.

Note: The definition of code P0126 may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What Are the Possible Causes of the P0126 Code?

In vehicles equipped with on-board diagnostics (OBD), trouble codes are set when the vehicle’s computer detects a potential issue within the vehicle. These codes may only indicate the nature and location of the problem but not the exact root cause.

There are a few reasons why the P0126 code is stored. Listed below are just a few of these possible causes:

What Are the Common Symptoms of the P0126 Code?

broken car heater
A heater not working is a common symptom of the P0126 code.

Once a trouble code is set, the vehicle’s computer may send a signal to illuminate the check engine light. In many cases, this may be the only indicator of a potential issue. However, other diagnostic trouble codes may manifest other symptoms. Below are some of the common symptoms you may notice if you’re dealing with the P0126 code:

  • Malfunction indicator lamp (MIL) or check engine light on
  • Abnormal temperature gauge reading
  • Heater doesn’t work
  • Vehicle stuck in “limp” mode
  • In some cases, fuel consumption may increase
  • In some cases, there are no noticeable symptoms present

How to Diagnose the P0126 Code

As mentioned above, there are many possible reasons why a trouble code is set. This makes diagnosis and repair time-consuming and, at times, difficult. In fact, there is a possibility that you may misdiagnose the issue. This may lead to more expensive repairs in the future.

For the P0126 code, many misdiagnose the P0126 code as being caused by a faulty engine cooling fan or an internal engine problem. Others may even conclude that the problem is a faulty engine coolant temperature sensor without conducting a thorough diagnosis.

Before attempting to diagnose the problem, you may want to look at technical service bulletins (TSBs) specifically addressing the trouble code you’ve encountered. These service bulletins contain repair information for widespread issues among certain models from the manufacturer.

If you’re planning to troubleshoot the P0126 code, you may want to invest in a vehicle-specific repair manual or database. These sources have detailed repair information, diagrams, and illustrations to help you better understand the issue at hand. However, if you don’t have access to these materials, you may also refer to how-to videos, blogs, and digital magazines for more information on trouble codes.

How to Fix the P0126 Code

Achieving an effective and long-term solution to trouble codes relies on two things. First, you need to accurately determine the root cause of the issue. This is a challenge for many as there are multiple reasons why a trouble code is stored.

Second, you have to consider that vehicles are not made the same. Terminology, components, and repair instructions will vary depending on the manufacturer. The solution to the P0126 on a Mazda 3 may not work to address the same issue on a Toyota vehicle.

That being said, repair manuals and databases may help make troubleshooting easier and more manageable. However, if you’re unsure about how trouble code repairs should go or if you’re not confident in your repair skills, you can always have a certified mechanic or technician do the troubleshooting for you.

Other Notes About P0126

If you own a Mazda, the code P0126 may seem familiar to you. That’s because this is a common issue in some Mazda models. In fact, Mazda has released a technical service bulletin to address the problem.

mazda cars lined up
If you own a Mazda, the code P0126 may seem familiar to you. Image credit: Mino Surkala

The Mazda technical service bulletin SA-035/20 was released on March 11, 2020 in order to address the P0126 engine code. The specific models affected by this problem are the 2018-2020 Mazda 6 (non-turbo) and the 2018-2020 CX-5 (non-turbo).

The bulletin suggests the replacement of the engine coolant control valve as a possible solution. However, there may also be other potential physical problems present, so you should not rule out other possible causes immediately. Proper diagnosis is still advised if you encounter the engine code P0126 on other makes and models.

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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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