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P0128: Coolant Temperature Below Thermostat Regulating Temperature

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    This story was created with generative AI and was reviewed and fact-checked by our staff of editors and experts.
    edited by
    Director, Editorial Content
    Steven Ewing has worked in the automotive industry since 2003. In that time, he's written thousands of car-related articles and tested just as many vehicles. Steven is Edmunds' director of editorial content and has previously been on staff at Winding Road magazine, Autoblog, Motor1.com, CNET Roadshow, and was the U.S. correspondent for Top Gear magazine. Steven has also contributed to Automobile magazine, Ars Technica, Car and Driver, The Drive, Jalopnik, SlashGear, and dozens of other publications. In his spare time, Steven loves to play the drums, cook, and lust over small JDM cars that never made it to the States.

Photo: Pav_1007 | iStock via Getty Images

What does the P0128 error code mean?

The P0128 error code is officially known as "Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)." This diagnostic trouble code is triggered when your vehicle's engine control module detects that the engine’s coolant temperature is below the expected range set by the manufacturer for optimal operation. Essentially, it indicates that the engine is not reaching its proper operating temperature within a certain time frame, which is often due to a malfunctioning thermostat or a fault in the coolant temperature sensor.

Why is the P0128 issue important?

This issue is critical because the engine operating at a lower-than-optimal temperature can lead to increased fuel consumption, higher emissions, and potential long-term engine damage. Engines are designed to operate efficiently at a manufacturer-specified temperature. When the temperature is too low, it can cause the oil to remain thicker, reducing lubrication efficiency and increasing wear on engine components. Additionally, the fuel combustion process may be less efficient, leading to incomplete fuel burn and increased emissions.

Can I safely drive my car with this error code?

Driving with a P0128 error code for a short distance is generally safe, but it's not a good idea to ignore the issue for an extended period. While your vehicle might still operate, it's likely to experience reduced fuel efficiency and potentially more serious mechanical issues over time. It's best to address the problem as soon as possible to avoid the risk of costly repairs.

Common symptoms associated with the P0128 error code

  • Lower than normal engine temperature readings
  • The heater not working effectively
  • Increased fuel consumption
  • Check engine light is illuminated

Diagnosing the P0128 error code

A mechanic will first verify the error code with an OBD-II scanner, then begin checking related components, including:

  • Inspect the coolant level and condition, looking for any signs of leaks
  • Test the coolant temperature sensor’s operation and its wiring for any faults
  • Check the thermostat’s operation, often by observing if the engine reaches and maintains the correct operating temperature
  • Inspect the engine control module for any signs of issues that could trigger this code in error

Repairing the P0128 error code

Repairing a P0128 error code often involves one or more of the following steps, depending on the root cause of the problem:

  • Replacing a faulty thermostat that is stuck open and doesn’t allow coolant to reach proper temperature
  • Repairing or replacing the coolant temperature sensor if it's found to be faulty
  • Topping up or replacing engine coolant if levels are low or if the coolant is in poor condition
  • Checking and repairing any electrical connections to the thermostat or coolant temperature sensor
  • Updating or repairing the engine control module software if it's contributing to the issue

Addressing this error code promptly helps ensure your vehicle operates efficiently and remains reliable over time.

Edmunds.comby

This story was created with generative AI and was reviewed and fact-checked by our staff of editors and experts.

Steven Ewingedited by

Steven Ewing has worked in the automotive industry since 2003. In that time, he's written thousands of car-related articles and tested just as many vehicles. Steven is Edmunds' director of editorial content and has previously been on staff at Winding Road magazine, Autoblog, Motor1.com, CNET Roadshow, and was the U.S. correspondent for Top Gear magazine. Steven has also contributed to Automobile magazine, Ars Technica, Car and Driver, The Drive, Jalopnik, SlashGear, and dozens of other publications. In his spare time, Steven loves to play the drums, cook, and lust over small JDM cars that never made it to the States.

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