What Does the Engine Code P0128 Mean?

If your car is showing the P0128 engine code, you’re dealing with a coolant thermostat malfunction.
Written by Tiffany Leung
Reviewed by Melanie Reiff
If your car is showing the P0128 engine code, you’re dealing with a coolant thermostat malfunction. This issue can greatly impact your fuel economy and emissions. Prolonged coolant temperature problems can also lead to serious damage to the engine.   
Diagnostic trouble codes (DTCs) give you a good indication of what issues your car’s engine is experiencing. If you have an OBD-II code reader, you’ll have a head start on identifying and evaluating the problem—and how to fix it. 
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What does the engine code P0128 mean?

Definition: Coolant thermostat malfunction—coolant temperature is running below the regulating temperature
The P0128 engine code indicates a problem with your car’s coolant thermostat, which is responsible for ensuring the engine warms up quickly enough after ignition to control emissions and fuel efficiency.

How much will it cost to fix?

Running a diagnostic for a P0128 will cost around one hour of labor, which may run you between $75 and $150.
If you need to replace the thermostat, the total cost for parts and labor will be around $470 to $505.

What can cause the P0128 engine code?

If your car is showing the P0128 engine code, there is likely an issue with a leaking cooling system or thermostat. A malfunctioning coolant thermostat is the most common cause of this engine code.
The coolant thermostat is responsible for blocking the coolant from entering the radiator until the engine reaches the correct temperature. This ensures that the engine can reach the correct temperature within a set timeframe to reduce emissions.
Other potential issues that could cause this engine code include: 

Common symptoms of the P0128 engine code

You may not be able to tell something is amiss with your coolant thermostat aside from the
check engine light
turning on. Other potential signs to watch for include: 
  • Higher than usual idle
  • Reduced fuel efficiency
  • Temperature gauge remaining low
If your thermostat is malfunctioning, you will also notice that your vehicle is producing more emissions than usual. 

How serious is the P0128 engine code? 

The P0128 engine code is not a critical problem, but you’ll still need to go in for an inspection. If the engine continues to run below the optimal temperature, it could become seriously damaged, necessitating costly repairs.

Can I fix the P0128 engine code myself?

In some cases, you can fix the P0128 engine code at home, but you should only begin repairs if you can diagnose the issue correctly. Depending on the issue, you may have to do the following:
  • Refill coolant
  • Replace
  • Replace coolant temperature sensor
However, if you’re not sure what repairs your car needs, bring your car to a professional to complete a thorough diagnostic inspection and repairs.

Repairing the P0128 code

If you do decide to repair the P0128 code by yourself, you can follow these steps:
  • Identify what is causing the code to show up and follow the below steps as required.
  • Replacing the ECT sensor
  • Inspect the ECT sensor near the thermostat
  • Remove and test the ECT sensor with a multimeter to ensure the correct resistance
  • Replace the ECT sensor if you cannot get a reading
  • Replacing the thermostat 
  • Remove the thermostat from the cooled engine
  • Install a new thermostat correctly, with the short end pointing towards the hose and the deep end pointing towards the engine. The jiggle valve should also point straight up to the engine to allow air through the system
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