DTC P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat)

P0128 – Coolant Thermostat

The P0128 code is a diagnostic trouble code (DTC) that indicates a malfunction in the coolant thermostat, specifically a coolant temperature below the thermostat regulating temperature. This means that the engine coolant temperature is not reaching the optimal temperature within the set time, which can lead to reduced engine performance and potentially cause damage over time.

How to Fix

  1. Check the Coolant Level and Type
    • Ensure that the coolant level is at the recommended level and that the correct type of coolant is being used.
    • If the level is low, add the recommended coolant and drive the vehicle for a short period to circulate the coolant.
  2. Inspect the Thermostat
    • If the coolant level is correct, inspect the thermostat for any signs of damage or malfunction.
    • If the thermostat is faulty, it should be replaced.
  3. Inspect the Coolant Temperature Sensor
    • Check the coolant temperature sensor for any signs of damage or malfunction.
    • If the sensor is faulty, it should be replaced.
  4. Inspect Other Components
    • Inspect the cooling fan, intake air temperature sensor, and other components for any signs of damage or malfunction.
  5. Consult a Professional
    • If the issue persists after inspecting these components, it is recommended to consult a professional mechanic for further diagnosis and repair.

Possible Causes

  • Faulty or worn-out thermostat
  • Cooling system flow issue
  • Cooling fan (stuck ON)
  • Incorrect engine coolant
  • Engine coolant leak
  • Faulty engine coolant temperature sensor
  • Faulty intake air temperature sensor
  • Sensor and ECM wiring issues (corrosion/open circuits)
  • Faulty ECM (rare)

Location of the Problem

  • The location of the problem is primarily in the cooling system of the vehicle, specifically in the thermostat and its associated components.


  • Check Engine Light illuminated
  • Engine doesn’t reach normal operating temperature
  • Reduced fuel economy
  • Reduced cabin heating
  • Slightly reduced acceleration/power

Possible Expenses to Fix the Problem

  • Thermostat replacement: $50-$200
  • Coolant temperature sensor replacement: $20-$100
  • Cooling fan replacement: $50-$200
  • Intake air temperature sensor replacement: $20-$100
  • ECM reprogramming: $80-$120
  • Labor costs: $100-$500

Please note that these are estimated expenses and may vary depending on the specific vehicle and the location where the repairs are performed.

[1] https://www.way.com/blog/p0128-code-causes-symptoms-and-how-to-fix-it/
[2] https://carbuzz.com/car-advice/p0128-code-what-it-is-and-how-to-fix-it/
[3] https://www.autozone.com/diy/diagnostic-trouble-codes/what-is-trouble-code-p0128
[4] https://www.samarins.com/diagnose/p0128.html
[5] https://www.purefjcruiser.com/docs/2007%20Toyota%20FJ%20Cruiser%20Repair%20Manual/1GR-FE%20Engine%20Control%20System/SFI%20System/0050027.pdf
[6] https://www.yarisworld.com/forums/showthread.php?t=54698
[7] https://www.carparts.com/blog/p0128-coolant-temperature-below-thermostat-regulating-temperature/
[8] https://www.kbb.com/obd-ii/p0128/
[9] https://www.youtube.com/watch?v=5PMszxut2Bw
[10] https://www.youtube.com/watch?v=AfDXh4cko84
[11] https://www.toyotanation.com/threads/check-engine-light-p0128.1751905/
[12] https://www.youtube.com/watch?v=s39kdXXE9YI
[13] https://www.toyotanation.com/threads/p0128-check-engine-light-code.242423/
[14] https://www.toyotanation.com/threads/p0128-coolant-temperature-below-thermostat-regulating-temperature-v6-3-0l.1688931/
[15] https://www.toyotanation.com/threads/p0128-causes-and-related-questions.1455706/
[16] https://www.youtube.com/watch?v=OoQwSodDgCE
[17] https://www.justanswer.com/toyota/3cz75-2000-solara-v6-failed-smog-check-p0128-failure.html
[18] https://www.justanswer.com/toyota/kzcmz-code-p0128-temperature-sensor-thermostat-2018.html

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