P0128 - Thermostat Rationality error 2012 JKU

Amrth

Amrth

Member
Hi All - A couple of weeks ago the check engine light came up, while I was on the highway.. Quick glance to the engine temp, showed it had not reached its optimal temperature. Reached back home and took a look in the diagnostics, I was able to get the P0128 - Thermostat Rationality error code. Looked up online and the results stated showed it might be a faulty thermostat or any leak in the coolant line.. I went ahead and cleared the DTC's and for a couple of weeks I let the engine heat up before I take off and I've observed it takes at-least ~10-15 mins on idle for the temp to reach at mid level. Again the light popped back up last night and I want to get this rectified.

I'm strongly planning on changing the thermostat, because I'vent found any leaks in coolant and a search on how to do it brings me to WAYLIFE thread, which make me more happier than ever. Thanks to @scull20 for this write up, so before I land up with this job, is there anything else that I should be looking at to rule out other possible causes?
also how soon should i get this fixed, I mean now I really wait till the engine temp gets to mid to start my drive, so I'm assuming at least this work around will not have an adverse affect on the engine right?

http://wayalife.com/showthread.php?35551-3-6L-V6-Thermostat-Housing-Replacement

Cheers,
 
It sounds like your tstat is stuck open which won't hurt your engine, but will take your heater longer to start working. If you get a laser thermometer gun, you can shine it on each side of the tstat and see if it's stuck open or closed.
 
WJCO said:
It sounds like your tstat is stuck open which won't hurt your engine, but will take your heater longer to start working. If you get a laser thermometer gun, you can shine it on each side of the tstat and see if it's stuck open or closed.
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Based on the motor not getting to temp I'm gonna say it's a stuck open thermostat and allowing the engine to "over cool"
 
WJCO said:
It sounds like your tstat is stuck open which won't hurt your engine, but will take your heater longer to start working. If you get a laser thermometer gun, you can shine it on each side of the tstat and see if it's stuck open or closed.
Click to expand...

Thanks WJCO.. I might have to check with the members of local club if they have one that I could use or look at renting it from a auto-store if they have it. In both the cases a replacement is inevitable, am I correct?
 
Amrth said:
Thanks WJCO.. I might have to check with the members of local club if they have one that I could use or look at renting it from a auto-store if they have it. In both the cases a replacement is inevitable, am I correct?
Click to expand...

You don't need a gun to test the heat, just use your hand. The upper house should be considerably hotter than the lower hose. If they feel about the same temp it's stuck open. If the upper hose is colder than it's stuck closed
 
Amrth said:
Thanks WJCO.. I might have to check with the members of local club if they have one that I could use or look at renting it from a auto-store if they have it. In both the cases a replacement is inevitable, am I correct?
Click to expand...

They're 25 bucks at a hardware store and a general good tool to have. If the tstat is stuck open, it's not going to hurt your motor, HOWEVER, your fuel injection system is designed to run based on the ECT input, so it could affect mpg and engine performance if the PCM isn't getting an ideal reading from the ECT sensor. If tstat is stuck closed, you would have already noticed major overheating concerns, it happens quick. Also for me, I replace the tstat on all my vehicles around 30k to 50k just as a preventative measure. For such a cheap part, it's worth it to me to not have it stick randomly.
 
cozdude said:
You don't need a gun to test the heat, just use your hand. The upper house should be considerably hotter than the lower hose. If they feel about the same temp it's stuck open. If the upper hose is colder than it's stuck closed
Click to expand...

Did this test earlier and that's one of my reasons to go for a replacement. Thank you.

WJCO said:
They're 25 bucks at a hardware store and a general good tool to have. If the tstat is stuck open, it's not going to hurt your motor, HOWEVER, your fuel injection system is designed to run based on the ECT input, so it could affect mpg and engine performance if the PCM isn't getting an ideal reading from the ECT sensor. If tstat is stuck closed, you would have already noticed major overheating concerns, it happens quick. Also for me, I replace the tstat on all my vehicles around 30k to 50k just as a preventative measure. For such a cheap part, it's worth it to me to not have it stick randomly.
Click to expand...

Sound like my plan of replacement is finalized, really appreciate your responses and time to order a OEM part and get on with it.

Cheers'
 
Thread revival!

Finally got around to pulling the code for the CEL I've had for nearly 2 years. I wasn't too worried because it would go off on its own after awhile in warmer temps. I was thinking gas cap. Turns out its P0128 which would explain why it'd go off when it was hot outside and come on when it was cold outside. Any recommendations on a tstat? Should I worry about the coolant temperature sensor?

Edit: Any tips or tricks on replacing it?
 
jesse3638 said:
Thread revival!

Finally got around to pulling the code for the CEL I've had for nearly 2 years. I wasn't too worried because it would go off on its own after awhile in warmer temps. I was thinking gas cap. Turns out its P0128 which would explain why it'd go off when it was hot outside and come on when it was cold outside. Any recommendations on a tstat? Should I worry about the coolant temperature sensor?

Edit: Any tips or tricks on replacing it?
Click to expand...
it's a one-piece housing and thermostat unit, I used the Dorman brand only because I've had good results with other Dorman parts.

It's an easy job, a bucket to catch the coolant when removing the hose from the housing or if you decide to unbolt the thermostat housing first, have a bunch of spill mats on the ground.

There is a petcock to release antifreeze from the radiator but it's a PIA to access and doesn't take many turns before it pops out and then a bigger PIA to put it back in place.

refill the radiator with the front end of the Jeep elevated to help remove air bubbles and one of these also makes it easier

1762650769249.png
 
jeeeep said:
it's a one-piece housing and thermostat unit, I used the Dorman brand only because I've had good results with other Dorman parts.

It's an easy job, a bucket to catch the coolant when removing the hose from the housing or if you decide to unbolt the thermostat housing first, have a bunch of spill mats on the ground.

There is a petcock to release antifreeze from the radiator but it's a PIA to access and doesn't take many turns before it pops out and then a bigger PIA to put it back in place.

refill the radiator with the front end of the Jeep elevated to help remove air bubbles and one of these also makes it easier

View attachment 434031
Click to expand...
Ok cool. Thanks for the info. It seemed pretty straightforward. I'll also take a class at YouTube University so I can get my expert cert...haha.
 
Not to mention when I do this I should probably do a flush and fill as well. 2012 with 87k on it and I've not done a flush. Just added OEM HOAT when needed. Coolant still looks good but it may not be doing its job properly.
 
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