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 26

Noreen Garland

July 27, 2018

Thank you

john chavez

December 3, 2018

awesome, very much thank you

Kimberly K Anderson

March 28, 2019

I have replaced everything on my dodge dokota 2004 with a 3.7 . 4 door and it is still giving me po120 code. Idk whatelse to do. I have checked all wires also Can not find the problem . And no one else including 3 machanics can find the problem. Help

Tina Little

August 27, 2018

Excellent information


September 21, 2018

Excellent explanation.


August 30, 2019

Problem solved itself Thanks


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

i am not a spokesman or a fixd employee. i am a customer that is very happy with a product i bought and that is the fixd sensor. Not only did it diagnose my BMW 5 series, but it warned me before the BMW light came on. i thought it was a defective product but my BMW warning light came on a day later confirming it. Wow. Then i was checking my email and there was my diagnose and fix in detail. Now that is customer service. Most firms forget you after you’ve bought the product. This company continued support and is helping me monitor my car. Awesome. BUY THIS PRODUCT everyone if you love your car. i love my car.
Ed H
San Jose, CA

Dr Jeffrey S Rise PhD

May 12, 2019

Nice comment, Ed, & you are exactly right. Fix’d seems to be a great company oriented towards excellent customer service….I am constantly fixing the cars, I buy & sell, & Fix’d makes my life much easier….

Leland Pannell

November 9, 2018

I love this fixd device. Best money I’ve spent in a while. Use it just one time and you may have paid for it.

Jorge Castro

November 17, 2018

Love this . help me a lot when im in the road.

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 ?


January 14, 2019

Thank you!

William Schultz

January 14, 2019

I’ve been aware of this P1028 code for about a year. Is it okay to just leave your scanner plugged in all the time? I’ve been doing this and just use your app to turn off the CEL if it comes on.

FIXD Marketing

January 15, 2019

Hey William! You can leave FIXD plugged in all of the time as long as you are consistently driving your vehicle. This being said, if the code is consistently coming up, we recommend that you try repairing it in order to fix the problem!


Geoffrey Namara

February 10, 2019

Thanks FIXD,
Happily using the device too. My 2004 lexus rx 330 brings up the check engine light along VSC and TRACK OFF lights at the same time consistently when i hit the high way (>80mph) within 10 mins of starting in the morning. And yes the radiator fan is constantly on. If this issue is around coolant temperature sensor or thermostat(FIXD shows P0128), what has it got to do with these other lights whose area should be around the tyres. Thanks

Kimberly K Anderson

March 22, 2019

I have a 2004 dodge dakota with a po128 code and a small evap leak code . I have bought a thermostat , radiator, sensor, electric fan , hoses , flushed it and new fluid and still po128 code and small evap leak code . or and new purge valve and gas cap to. Done smoke text checked vent valve or charchole canister and all hoses and wires. I can not fix it and neighther can two machanics figure it out. I can not afford another machanic again. Please help me ! My insprction in due by end of march 2019 and my tags to ! HELP ! bkimmiek69@yahoo.com


April 15, 2019

Did you reset the warning light?


August 15, 2019

Ahhh the dreaded P0128 code….
I ended up using an AST Smart ECO VCM Module to help my 2008 Honda Accord that was having spark plug issues. http://www.FixMyV6.com
Apparently lots of Honda vehicles with the V6 ECO engine have problems from ECO mode, and this greatly helps it.


October 7, 2019

I replaced thermostat in my 2003 toyota tundra because i received code P0128.. The check engine light is still on after replacement and the heater does not blow out hot it is cold as if i have the AC on..

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