P0128 Code – Meaning, Causes, Symptoms (& How To Fix)

P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

P0128 Code

The engine must run at optimal temperatures for efficiency and performance. When it can’t, you might see the Check Engine Light come on and the P0128 code in your scanner, but what does this mean?

In this guide, we cover the meaning of the P0128 trouble code. We also examine what went wrong, show you the symptoms it might exhibit and give you some tips to fix it. 

Code Definition

P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What Does the P0128 Code Mean?

Code P0128 indicates that the engine coolant hasn’t gotten hot enough in the appropriate timeframe. The Engine Control Module (ECM) is responsible for monitoring the ambient temperature and determining if it is in line with manufacturer specifications. 

To determine if the engine has reached normal operating temperatures, the ECM examines the length of time that the vehicle is running. It also looks at the speed of the vehicle, the engine coolant temperature (ECT) sensor and the intake air temperature (IAT) sensor reading. 

P0128 Trouble Code Symptoms

The most common symptom with the P0128 trouble code is the inability to get the engine temperature up to normal operating specifications. There aren’t many symptoms that this condition will cause, making it simple to pinpoint the problem.

Here are the most frequent symptoms occurring with P0128:

  • Engine doesn’t reach normal operating temperature
  • Engine takes longer to reach normal operating temperature
  • Motor temperature drops down while driving at high speeds. 

Causes of the P0128 Code

The most common cause of the P0128 trouble code is a stuck open thermostat. This part is needed to control engine temperature. However, there are other possible conditions you should consider.

Here are the most popular causes:

How Serious is the P0128 Code?

Medium – Having an engine that doesn’t reach normal operating temperatures isn’t nearly as dangerous as one that overheats. However, you don’t want to overlook the damage that can be done when operating an engine that won’t reach proper temperatures.

You should have the fault repaired as soon as possible. Extended use of an engine that’s too cold can lead to performance issues and a drop in fuel economy. In the end, it can create engine damage, which is expensive to repair. 

What Repairs Can Fix the P0128 Code?

In the majority of cases, the thermostat needs to be replaced to fix the P0128 trouble code. However, there are multiple repairs that could fix the fault. That’s why it’s vital to perform an in-depth diagnostic examination before deciding on the repair. 

Here are a few of the possible solutions:

  • Replace defective thermostat
  • Top off engine coolant level
  • Replace cooling fan 
  • Replace coolant temperature (ECT) sensor
  • Replace intake air temperature (IAT) sensor
  • Repair/replace wiring 

RELATED: 6 Causes of a Cold Lower Radiator Hose (& How to Fix it )

Common P0128 Diagnosis Mistakes

The majority of mechanics will agree that the most obvious cause of P0128 is a defective thermostat. In fact, it’s considered the most frequent repair for the fault. 

However, you don’t ever want to jump to conclusions and replace the thermostat without a complete diagnosis. There are times when a thermostat won’t fix the trouble code, so it’s important to ensure it needs to be changed first. You want to ensure the coolant level is good first of all.

Recommended Tools for Diagnosis

How to Diagnose the P0128 Trouble Code

Here are the steps that a professional mechanic will take to diagnose the P0128 engine code.

  1. Start with the engine coolant thermostat. Cool the car down completely. Start the engine and put your hand on the radiator hose. Use caution because this hose should get very hot. If it doesn’t get warm or it warms up slowly, the thermostat is to blame. Be aware of any moving parts, which can result in serious injuries!
  2. One of the hoses from the radiator, should be cold until the engine temperature reached full temperature, and then slowly open. If both hoses are the same temperature, it could be a thermostat stuck open.
  3. Double-check the coolant level and condition, as contaminated coolant can cause a sticking thermostat.
  4. You can check the coolant levels even if the hose gets hot right away.
  5. Listen to the cooling fan to see if it is stuck on.
  6. Check the coolant temperature sensor with your scan tool. You should take a manual reading and compare it with an infrared thermometer. The typical temperature should be around 200-degrees Fahrenheit.
  7. If these readings don’t match, you need to inspect the connectors and wiring. 
  8. Once you repair the problem, clear the codes. Take the car for a test drive and see if the code comes back. 

Estimated Cost of Repair

Depending on what needs to be fixed, you have a variety of repair costs. These estimates assume you need to pay for the part and labor. 

  • Replace defective thermostat – $450-$600
  • Top off engine coolant level – $15-$45
  • Replace cooling fan – $550-$700
  • Replace coolant temperature (ECT) sensor – $150-$300
  • Replace intake air temperature (IAT) sensor – $75-$150
  • Repair/replace wiring – $50-$1,500

Mechanics Tips about the P0128 Code

When you work with the P0128 code, you might be tempted to get right in there and get the diagnostics done. However, you must always use caution and allow the engine to cool off. With the engine warm, the coolant is pressurized. It can cause severe burns because of how hot it is.

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