P0128 – Meaning, Causes, Symptoms, & Fixes

Code P0128 Definition

Coolant thermostat temperature below regulating temperature

Code P0128 Meaning

The powertrain control module (PCM) tracks how long it takes for the engine of a vehicle to reach and maintain the correct operating temperature. When the correct engine operating temperature is reached, the powertrain control module orders the fuel system to enter “closed loop” where readings from the oxygen sensors are used to maintain the efficient air to fuel ratio of 14.7:1. Most PCMs mandate that the engine coolant temperature sensor record temperatures above 160º F within 15 minutes of the engine starting. Additionally, once the 160º F threshold is achieved, the recorded engine temperature must not fall below 160º F during operation. The PCM will record if either of these two criteria are not met. If either fault is recorded again on the next engine startup, code P0128 is triggered.

P0128 Symptoms

  • Check Engine Light is on
  • Higher than normal idle
  • Decreased fuel economy
  • Temperature gauge is unusually low

P0128 Causes (* indicates most common)

  • Stuck open thermostat*
  • Faulty coolant temperature sensor
  • Faulty wiring for coolant temperature circuit
  • Radiator fan constantly running

Code P0128 Severity – Low    

A stuck open thermostat is most likely the problem, however thermostats are usually designed so when they fail they are stuck open allowing full flow of the coolant protecting your engine. Check your coolant level first to make sure it is full before continuing to operate the vehicle. Overheating your engine will result in engine failure

Code P0128 Common Diagnosis Mistakes

Many just replace the thermostat without checking all possible causes. Check the condition of the cooling system for rust deposits or mixture of different coolants. Make sure to flush the engine block and radiator when replacing thermostat to prevent deposits from getting stuck in the new thermostat.

Code P0128 Diagnosis Steps

How To Diagnose P0128

  1. Scan your vehicle to verify P0128 is the only code present. If other codes are present, they must be addressed first.
  2. Check coolant level and condition. If there is excessive rust and poor coolant condition this can clog up the cooling system or cause the thermostat to stick. If your coolant condition is poor, flush the coolant system and replace the coolant. If the coolant level is low, fill the coolant system and check for leaks.
  3. The coolant temperature sensor can be checked with a multimeter. The ohm reading will change with the temperature. If the ohm reading is not changing with the temperature, replace the coolant temperature sensor or repair the wiring for the sensor if it is damaged.
  4. The most common cause for P0128 is the engine coolant thermostat is stuck open. A simple way to diagnose this is to feel the radiator hose and monitor how hot the temperature of the coolant is when it starts flowing through the radiator hose, however, you should be extremely careful when doing this as you could be burned. The hose should be barely warm until the thermostat opens. When the thermostat does open, hot coolant should start to flow and quickly warm up the radiator hose. If the radiator hose heats up slowly, the thermostat is stuck open or opening prematurely and needs to be replaced.
  • Replace thermostat with new gaskets and coolant
  • Test drive and monitor coolant temperature
  • Check for leaks
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Comments 15

Noreen Garland

July 27, 2018

Thank you

john chavez

December 3, 2018

awesome, very much thank you

Tina Little

August 27, 2018

Excellent information


September 21, 2018

Excellent explanation.


September 24, 2018

Thank you for your help. would oil not being changed cayse this also?

Robert Harrison

September 28, 2018

good, straightforward info, for which many thanks!
BUT – what drive cycles (or whatever they are called) would it take to reset the computer after it starts reading the right temperature?
(I have a 2005 Chevy Astro; check engine light on; P0128 code on scanner; temperature clocks at 160 instead of 195)

Edward Howard

October 9, 2018

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November 9, 2018

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Jorge Castro

November 17, 2018

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FIXD Marketing

November 17, 2018

Thanks Jorge! Glad we can help!


Kim Anderson

November 23, 2018

I love this divice ! It has help me countless time to figure out my problems fixing my truck. Saves me so much money and time !

FIXD Marketing

November 27, 2018

Awesome Kin! Glad we can help!



November 27, 2018

My 2005 Sienna has 192K miles and the “check engine” came on. The scanner showed the error code P0128. If I don’t get it fixed right away, what damage it might cause to the car other than a decrease in vehicle performance and fuel mileage? I am debating how much money in repairs I should invest considering the mileage of the vehicle. Much thanks.


December 2, 2018

How about an answer to Jenny from November 27, 2018. Good question!

Also. . . . after my diagnosis of P1028 when I click to remove engine warning light it says there is no scan device. What is that all about?


December 11, 2018

The scanner showed PO128, the gadget isn’t moving and the temperature shows 0 . It don’t smell like it over heating so i’m thinking i need to replace my thermostat . What do you think ?