Air & Fuel Mixture ControlOBD2 Codes

P0128 – Meaning, Causes, Symptoms, & Fixes


What Does Code P0128 Mean?

  • P0128 definition: Coolant Thermostat Temperature Below Regulating Temperature
  • Issue Severity: LOW– Continued driving for a short period of time is okay.
  • Repair Urgency:  Get this code fixed withing the next month to prevent . 
  • Diagnosis: It is important to complete the entire diagnostic process when diagnosing the P0128 code. This DTC can be triggered by a missing or stuck open thermostat, a faulty coolant temp sensor, or damaged wiring to the coolant temp sensor circuit.

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The engine control module (ECM) 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 a “closed-loop” where readings from the oxygen sensors are used to maintain the efficient air to fuel ratio of 14.7:1. Most ECMs 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 is not met. If either fault is recorded again on the next engine startup, code P0128 is triggered.

P0128 Causes

  • Faulty/stuck open thermostat (most common)
  • Missing Thermostat 
  • Faulty coolant temperature sensor
  • Faulty wiring for coolant temperature circuit

P0128 Symptoms

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

How Do I Fix Code P0128?

With a P0128 code, the first step is to get it properly diagnosed to figure out what is causing the malfunction in the coolant temperature circuit. Using the live data function of the FIXD sensor and app allows you to read and analyze engine temperatures to properly diagnose a P0128 code.

If your engine is running lean and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal-certified shop to pinpoint the problem and give an accurate estimate for repairs. 

These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.

> Find a RepairPal Certified Shop Near You

How Much Does It Cost to Fix Code P0128?

If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your P0128 code.

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  • Upfront cost estimates
  • Minimum 12-month, 12,000-mile warranty
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Possible Repair Costs for P0128

When it comes to making repairs associated with the P0128 code, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair. 

  • Loose/damaged gas cap
  • EVAP hoses
  • Fuel tank
  • Charcoal canister
  • Faulty vent control valve
  • Faulty purge control valve

DIY Steps to Diagnose Code P0128

If you’d like to try to fix code P0128 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Diagnosis can require some specialized equipment beyond what the FIXD Sensor can provide, but, on most vehicles, this is still a beginner-level diagnosis and repair for DIYers. 

DIY difficulty level: Easy

This repair is easy enough for beginner-level DIYers to attempt.

Tools/parts needed (our top picks from Amazon):


Use FIXD to scan your vehicle to verify P0128 is the only code present. If other codes are present, they must be addressed first.


Check the 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.


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.


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. Nevertheless, 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, the 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.


If at this point the vehicle is still setting the same code, you may have a more serious problem with your car’s engine coolant system, and you should bring the vehicle to a certified shop to have further diagnostic work performed.  

Common P0128 diagnosis mistakes

It is important to complete the entire diagnostic process when diagnosing P0128. Don’t overlook an easy cause such as a loose connector for the engine coolant temperature sensor or low and/or dirty coolant.

Still Need Help Fixing Code P0128?

If you’ve followed the steps above and are still experiencing issues with the coolant temp circuit and code P0128, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you to get the right repairs at a fair price.


Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a ’91 GMC Syclone. #GirlDad #SaveTheManuals

Jeffrey N. Ross
Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a '91 GMC Syclone. #GirlDad #SaveTheManuals

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  1. Thank you

    1. awesome, very much thank you

    2. 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

      1. Excellent information. That explains my problem. Going to replace both the sensor and thermostat as well.

      2. This was very informative thank you

      3. Make sure anytime you fix an issue that causes the check engine indicator to come on, you delete the code with the FIXD Sensor. Some vehicles you can unhook the ground and positive battery cables and touch them together to reset the code too.

      4. My toyota Avalon 2002 always case overheated while using Ac for a long time.

  2. Excellent information

  3. Excellent explanation.

    1. Problem solved itself Thanks

      1. Is it okay if i dont do anything and just reset it?i had the same issue 6 months ago replaced the battery as i was having issues with it.all worked fine.went to service was diagnosed but the mechanic didn’t say anything about the coolant or temperature.

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

  5. 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)

  6. 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

    1. 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….

    2. I have purchased the product fixed. But it’s not providing the information that i need to focus on the electrical problem.
      I got no ignition fire to crank sensors are replaced.
      New battery and pleanty of kick to the cables in motor compartment.
      I need help, love my car and thinking of junkyard time..

  7. 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.

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

    1. Thanks Jorge! Glad we can help!

      FIXD Team

  9. 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 !

    1. Awesome Kin! Glad we can help!

      FIXD Team

  10. 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.

  11. 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?

  12. 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 ?

  13. Thank you!

  14. 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.

    1. 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!

      FIXD Team

  15. 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

  16. 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 !

  17. Did you reset the warning light?

  18. 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.
    Apparently lots of Honda vehicles with the V6 ECO engine have problems from ECO mode, and this greatly helps it.

  19. 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..

  20. Had this code last year, Replaced the thermostat, same thing this year. Why?

    1. I have a Chevy impala 2008 and ive got the same issue. For the last 2 yrs ive replaced thermostat and temp sensor and now I’m getting the same code but i also have evap vent valve solenoid malfunction p0449 as well. So gonna try fixing the issue with the p0449 code, clear codes, then run and scan again to see what happens! Figures crossed! If nothing else, this pos car my fiance bought after i told him it wasn’t wise to buy first off, will be going for a nice Lil swim in a deep lake somewhere! Haha btw he knows nothing about cars, Hes a youtuber/gamer where-as I’m the mechanic and mom and home Repair person! Hahaha love doing it tho and getting my hands dirty and to have it all work after I’m done fixing the problems is highly satisfying i must say! 😁

  21. How do u get rid of Ck eng lite after replacing thermostat

  22. My check engine light, VCS and trak light was on. My mechanic tested it and found it was the P0128 code. I did not have it fixed but was going to take it back to get it fixed. But, then a few days later I turned on my heater and then all the lights went off. Does this mean there is nothing wrong with the thermostat. Perhaps something was stuck and now it is unstuck and everything is fine?

  23. I am a 64 year old woman that uses FIXD. When my check engine light went on, I looked at the codes and possible corrections. The problem had 3 different fixes. I ended up getting a new gas cap which removed 2 of the codes and the third was a CO2 sensor. I took my car to be evaluated showed the tech what sensor needed to be replaced. I saved myself $125 because the car did not need to be hooked up to the diagnostic scanner. Also love that I can clear the check engine light, the rating to the level of possible harm to the car and approximate cost for a repair. Best car gadget on the market.

  24. I installed an Thermostat in my Dodge Journey and think I have it fixed , but I’m having trouble finding how to reset the check engine light. I found in some of the instructions that referred to a details list, but am unable to find the same. Please instruct me as to how to reset the check engine light please! Thanks

  25. I am an over 70 year old woman. I have a 2003 Cadillac Ext truck that I purchased new. I am grateful that I have had very few problems over the years. Recently, I have had to spend a few dollars on general repairs. I saw and ad and purchased the FIXD a few months ago. This is such a wonderful gadget and I would recommend it to everyone, especially women. I had a check engine light come on this morning. I performed the scan and it gave me a code that I gave to my mechanic. Doing so, saved me the diagnostic fee. I am taking the truck in on tomorrow for the
    repair. I am going to send one of these things to my daughters. Thanks FIXD…….. I haven’t purchased the premium, but will be doing so this week. For women, and some of you guys too, this will be power and more knowledge on mechanical issues. I have already learned much more than I did before I purchased FIXD.

    1. My wife is using her nice car which is Dodge Charger. The engine light came on I bought the FIXD Sensor and used it on her car is shows the po120 and my wife change gas level up to 89 Octane. It gave her a better gas performance that’s also change sensor in the 3 days the engine light disappear, so the device is a love buddy to have!

  26. My wife is using her nice car which is Dodge Charger The engine light came on I bought the device and use it on her car is shows the po120 and my wife change gas level up to 89 Octane it gave her a better gas performance that’s also change sensor in the 3 days the engine light disappear, so the device is a love buddy to have

  27. Seems a bit expensive for a DIY. I purchased a thermostat for $12 and antifreeze for $14. I replaced the unit in under an hour. All the codes cleared after 30 minutes of driving. That was for a 2012 Dodge caravan.

  28. Love this product but not my car. 2006 Mercedes ML.50 with only 122,000 miles. P2006 and the P0128 code. Mechanic says it needs a new transfer case which will cost more than the car is worth. How do I dispose of a car that looks great but is not worth fixing?? My husband knows nothing about cars!!

  29. I have a 2010 Ford Ranger bought it new no I’ve been getting the check engine light coming on had the shop where I take it replace thermostat and housing and sensor and replaced fluid this is my 3rd time taking it to shop and tomorrow I’m taking it back there as the code comes up again as the thermostat also it says the tire pressure sensor but tires are all good getting Frustrated?

  30. FIXD saved me $235 by diagnosing p0128 code as stuck thermostat or low coolant. the dealer insisted the $250 thermostat replacement was the only way. $15 in premix later, problem solved, check engine light extinguished itself.

  31. I had a p0128 code on my 2016 Camaro. When i got that code my a/c didn’t work as well as the thermostat guage not working. I reset the warning light and all works fine. Is this normal when this code appears?

  32. code 128 high engine temperature is because my temperature gauge is not moving to throw the alarm. It has to be a connection at the computer, because when the code is cleared, the temperature gauge registers true engine temp. At first I replaced the sensor. Not sure if it is weather related, cold and moist, or just a skip in the engine start routine.

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