2013JKU error code and message P0128 "Thermostat Rationality".

CharlesC

CharlesC

Member
-Blend door actuator motor- mine went to shit on me last month. With a flash light look under the dash right behind the temp sensor on the driver side. Turn your temp dial and see if there is a white plastic arm that moves front to back and vise versa. If that is not moving, it is the motor that is causing the problem. Hope this helps!


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WJCO

WJCO

Meme King
A stuck open tstat will cause that code and will also cause the heat to take forever to get hot. So if you're a short distance driver, you likely won't even get the engine up to normal operating temperature.
 
TomDabb

TomDabb

Member
Awesome- thanks. The actuator located in the console was replaced last week - gave a little warm, 83 degrees at the registers, nothing like the 160 I measured last winter.
But I will check out the blend door actuator motor as well thanks for that advise.
I drive a lot of highway miles, engine temp gage straight up (whatever happen to Gage's w degrees, lol) the air temp is definitely warm when sitting still a while Than cools a bit on the drive (I guess that can be attributed to lack of closure on the T stat. Thanks great input- appreciate you answering me.


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TomDabb

TomDabb

Member
Thanks Jeepy for the informative info. I saw it with great discomfort this summer and hoped that wouldn't be me - tomorrow the thermostat comes out - I'll watch and see what else comes with it paying close attention to the coolant color especially.


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tdougherty81

tdougherty81

New member
TomDabb said:
Thanks Jeepy for the informative info. I saw it with great discomfort this summer and hoped that wouldn't be me - tomorrow the thermostat comes out - I'll watch and see what else comes with it paying close attention to the coolant color especially.


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Click to expand...

I just had this not to long ago. I replaced the thermostat and the coolant temp sensor. Figured it was cheap enough to replace both while the coolant was drained. This also fixed my driverside heat for some reason. Thought it was the blend door but after I did that it started to work.
 
TomDabb

TomDabb

Member
Still no real joy - or significant heat.
Replaced the actuator under the console and thermostat.
I think I need to check that actuator motor for the blend door next. What do think?


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TomDabb

TomDabb

Member
Follow up-
Replaced t-stat and blend door actuator. Heat is barely warm at best only during idle or very slow moving. Mechanic says it might be a clogged or dirty heater core. That's my next step, I think. Btw, 2013 JKU w 80k.
Any additional thoughts would be helpful thanks!


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WJCO

WJCO

Meme King
TomDabb said:
Follow up-
Replaced t-stat and blend door actuator. Heat is barely warm at best only during idle or very slow moving. Mechanic says it might be a clogged or dirty heater core. That's my next step, I think. Btw, 2013 JKU w 80k.
Any additional thoughts would be helpful thanks!


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Click to expand...

Try good chemical flush first. Replacing heater core is a pain in the ass.
 
TomDabb

TomDabb

Member
The latest!
Ok so I get warmed air driving below 30 mph. Once on the highway 55-75mph warm air is a wish. So, it got me thinking a bit more - Could I have a bad water pump?

Any thoughts?


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TomDabb

TomDabb

Member
2013JKU error code and message P0128 "Thermostat Rationality".

Third time is a charm! Yes Bmans08jk, you hit the nail on the head.
To sum up, replaced the blend door actuator - no change, T-stat - marginal improvement , finally reverse flush heater core.
Great! 160° at the registers.
That's a good benchmark if you have any doubt.
Thanks again for all your inputs.


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Last edited:
dpetersen1988

dpetersen1988

Member
You could have a leak in the cooling system somewhere as the engine is not getting warm enough and throwing that code, their is also the possibility that there is a clog in the system or air in the system that is throwing the pressure sensor off and causing the code to populate, if u know how to pressure test the radiator u could give that a try or have your mechanic do it. The pressure sensor is built into the temp sensor on the inlet according to my service manual.
 
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