What is wrong with my thermostat?


ivarr

Member
Joined
Dec 25, 2019
Messages
27
Location
massachusetts
so for 3 years now my engine light comes on when it starts getting cold and throws a p0128 code. but every spring it stops and i go through the entire summer with no check engine light annoying me.
{{ EDIT: correlates with starting or stopping use of the heat for the change in seasons. }}
i dutifully change the damn thermostat every year, last year i even wired in a new sending unit plug just in case. and last week the thing started it again. yes i can make it through life clearing the codes every few days, but i'd rather know what the hell is going on.
 

Irun

Jeep Addict
Supporting Member
Joined
Oct 31, 2019
Messages
4,261
Location
Virginia
Have you done a heater core flush and cooling system cleaning? You'd be shocked at the crap that comes out of both the heater core and block. Many years ago now, but I had a similar issue where crap in the cooling system would stick the thermostat. I've used this a couple times recently and it helps to effectively clean out rust.

 

pagrey

TJ Expert
Supporting Member
Joined
Apr 10, 2018
Messages
5,092
Location
Los Angeles, CA, USA
Multiple thermostats throwing the code seems to point to wiring, you might start with all the ground cables including the battery terminals. They should be really close to perfect. I guess flushing couldn't hurt but I'd personally be looking at wiring.
 
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JKP

TJ Enthusiast
Joined
Aug 20, 2020
Messages
384
Location
Warner Robins GA
That is a good thermostat. I replaced the original one with a Stant at about 170K or so when the rad blew. Now at 230K.

If the gauge is reading 210 or so, that is where it should be.

Have you checked to see if your fan clutch has seized?