Code P0128 - replaced thermostat

[DodgeaWrench]

[2014 DJ 3.6 177k miles]

 

Solid yellow check engine light came on.  Code P0128 - Coolant Temperature Below Thermostat Regulating Temperature.

 

This occurred once before, approximately 5 years ago, so I had a clue as to what was happening.

While taking a "test" drive, I toggled through the display to show coolant temperature.  Coolant temperature would not rise above mid-180 Fahrenheit.  (My understanding is that it normal should be above 200-ish).  Time for another thermostat.

 

As repairs go, replacing the thermostat is one of the easier repairs to do (2 10mm bolts and a spring clamp, easy access).  Having a spring clamp tool (on a cable type) makes it a breeze.

 

As mentioned, mine was replaced once before at 109K miles Jan-2020 $260.00 (repair shop, not dealer).  A Mahle brand thermostat was installed.  I went to a repair shop instead of doing it myself as it was the middle of winter and I do repairs in the driveway.  This time it is a beautiful fall weekend to take advantage of, with winter soon to knock on the door.  No time to order an OEM.  I went to the local auto parts store and purchased a Murray brand thermostat $19.00.

 

After the swap, the code was reset and a small test drive afterwards had the temps rising into the low 200's with the check engine light stayed off.

Next is to watch for leaks.  I'll post a follow-up in about a week.

 

 

 

   

 

 

The old thermostat in location

 

 

 

The new

[yyz]

Worst part I find is that stupid plastic bleeder screw.I replaced mine with a brass one.

[larryl]

I just reset mine I like it running cool....

[14WhiteRTRallye]

Mopar stat is a Mahle in a Mopar box. I found out the expensive way.

[Daniel Anderson]

Larryl what oil temperature are you getting

[larryl]

usually around the same as the coolant temp rarely over 205* 

I use 5-30 oil also approaching 210000 miles no consumption so far

[Daniel Anderson]

OK I decided at plus six Celsius today to replace thermostat and housing as it has been stuck at 85 Celsius all Summer  (code thrown). What I found out is that NAPA's replacement thermostat housing mounting holes and bolts are too big for my engine. (Dorman OE solutions Part No. 902_3036XD).My original (OEM) was 7 mm Dia.  (Included metal bushings inside plastic housing)  Now I was only able to use new  thermostat, rubber seal, locking bracket and spring from ordered part

C a u t i o n

Please compare sizes before swapping out

Edited November 13, 2024 by Daniel Anderson
Added supplier name

[Daniel Anderson]

8 minutes ago, Daniel Anderson said:

OK I decided at plus six Celsius today to replace thermostat and housing as it has been stuck at 85 Celsius all Summer  (code thrown). What I found out is that NAPA's replacement thermostat housing mounting holes and bolts are too big for my engine. (Dorman OE solutions Part No. 902_3036XD).My original (OEM) was 7 mm Dia.  (Included metal bushings inside plastic housing)  Now I was only able to use new  thermostat, rubber seal, locking bracket and spring from ordered part

C a u t i o n

Please compare sizes before swapping out

[5rebel9]

8 hours ago, Daniel Anderson said:

INTERESTING.....

My shop owner friend swears by the aluminum housing T-stats for ALL the 3.6 engined models. He has them on all his own and family's cars and EVERY taxi minivan fleet he services with NO PROBLEM.

With my own 3 3.6 motored DJ's, I;ve only replaced once each on our '11 and '13 with the plastic Dorman units for stuck open stat.  I wonder if I have just been "lucky" or something.

[Daniel Anderson]

Ya the bolts (too wide)

they gave me don't even fit the holes in the housing !

 

[5rebel9]

Without looking up the bolt spec.'s, I'm pretty sure the 2 bolts for the "stat" are 6mm, so the holes in the cover should be a little bigger. Are you sure they sold you the correct bolts? Your pic. can't tell definitely, but bolt looks to be an 8mm, and definitely would not be correct for engine or stat housing.

[Daniel Anderson]

I will remeasure for you, the bolts they gave in the box. My eyes are not good for vernier caliper after decimal. But for the two replacement bolts: 29.6 mm x 9.8 mm. (D 10.9 cap head)

Edited November 14, 2024 by Daniel Anderson
Missing letter T for he

[Daniel Anderson]

Thermostat housing hole size:  7 mm diameter 

[smfabac]

I'm floored to hear that the Journey will throw P0128 code!!!  I stumbled upon this post checking YouTube videos for thermostat replacement for my 2013 Journey Crew as the temperature today started at 22 Deg F and the car is very slow to warm up - reaching 168 Deg F after 17 miles. There is no P0128 code.  When the engine does not warm up, oil pressure does not drop so I see 77 psi while moving that drops back to 35 or lower stopped at a light. Since the engine is obviously running cold why is there no P0128 code?  

[5rebel9]

5 hours ago, smfabac said:

I'm floored to hear that the Journey will throw P0128 code!!!  I stumbled upon this post checking YouTube videos for thermostat replacement for my 2013 Journey Crew as the temperature today started at 22 Deg F and the car is very slow to warm up - reaching 168 Deg F after 17 miles. There is no P0128 code.  When the engine does not warm up, oil pressure does not drop so I see 77 psi while moving that drops back to 35 or lower stopped at a light. Since the engine is obviously running cold why is there no P0128 code?  

IF you have the O.E. plastic thermostat unit, the stat is stuck PARTIALLY open just enough to to prevent full warm up and yet not set the code. I had this happen on our '11 main street. Buy the aluminum replacement unit and be done with these type of concerns. The original bolts are used with the aluminum housing unit with NO problem as was posted by member Daniel Anderson. I also had thermostat "sticking" on our '13 rt but stuck closed this last summer with the PLASTIC type housing, luckily we shut the car down before damage was done and replaced it with the Aluminum housing type, no problem since. 

[larryl]

Mine sets the code but I just reset it rather have it run cool

[smfabac]

18 hours ago, 5rebel9 said:

IF you have the O.E. plastic thermostat unit, the stat is stuck PARTIALLY open just enough to to prevent full warm up and yet not set the code. I had this happen on our '11 main street. Buy the aluminum replacement unit and be done with these type of concerns. The original bolts are used with the aluminum housing unit with NO problem as was posted by member Daniel Anderson. I also had thermostat "sticking" on our '13 rt but stuck closed this last summer with the PLASTIC type housing, luckily we shut the car down before damage was done and replaced it with the Aluminum housing type, no problem since. 

Thanks for the answer.  I've considered the aluminum housing type available from NAPA (I like the NAPA AAA member discount), but am hesitant wondering if the radiator hose will adhere more tightly to the aluminum then the plastic of the OE housing.  What has been your experience?  Since I have not changed the thermostat in the last 12 years, at age 77 worrying about disconnecting a stuck hose from an aluminum housing  is likely not an issue.  

[smfabac]

14 hours ago, larryl said:

Mine sets the code but I just reset it rather have it run cool

I don't know what to say to that. How can resetting the code fix the problem making it run cool? 

[5rebel9]

4 hours ago, smfabac said:

Thanks for the answer.  I've considered the aluminum housing type available from NAPA (I like the NAPA AAA member discount), but am hesitant wondering if the radiator hose will adhere more tightly to the aluminum then the plastic of the OE housing.  What has been your experience?  Since I have not changed the thermostat in the last 12 years, at age 77 worrying about disconnecting a stuck hose from an aluminum housing  is likely not an issue.  

I have found NO DIFFERENCE in being able to seperate the hose, but I prefer using a screw hose clamp instead of the spring clamp as done O.E.. I find that "working room" to be more easy than trying to get pliers onto that area.

[larryl]

What I meant was that I don't worry about it running at 160 seems to make no difference to the engine and it has 225000 miles on it

Better than overheating

[smfabac]

4 hours ago, larryl said:

What I meant was that I don't worry about it running at 160 seems to make no difference to the engine and it has 225000 miles on it

Better than overheating

The problem I see is that oil pressure stays high until the engine reaches at least 180 Deg. On cold start I see max pressure driving away of 77 to 85 psi  dropping to about 35 psi stopped at idle. When the engine is properly warmed up to operating temperature idle psi is 20 to 24 and easy acceleration from a stop and running at 55 mph get to 40 to 50 psi. In the past I had a failed coolant  temperature sensor on the oil cooler so the temperature never increased and oil pressure continually ran at 77 to 85 psi until I had the oil cooler replaced (with all sensors pre-installed in the new unit). The service advisor at the previous oil change indicated that the filter/cooler housing is leaking. But I delayed until the oil pressure sensor failed (also on the filter/cooler housing P0520) and had to be addressed. The oil filter housing/oil-cooler was changed and new spark plugs installed at the same time at 114,750 miles December 2019.

 

I understand that engines are designed to run at an operating temperature of 212 to 220 deg F. Constantly running cold drops the gas mileage you could have got running at the designed temperature. 

[John/Horace]

I especially don't like the idea of running longer than you have to at a high psi with the plastic housing heat exchanger. I'm on my second one, no Dorman alumium option when first failed. It was in the middle of nasty January cold snap at around 102k miles 5yrs old. Filter cap always tightened with inch pound torque wrench, and always 5w20 synthetic oil. No crack found, base oring seals failure I think. Complete new oem...it was January. 

[smfabac]

On 1/21/2026 at 4:25 AM, 5rebel9 said:

I have found NO DIFFERENCE in being able to separate the hose, but I prefer using a screw hose clamp instead of the spring clamp as done O.E.. I find that "working room" to be more easy than trying to get pliers onto that area.

I purchased the aluminum DORMAN 902-3036XD at NAPPA and installed it yesterday. My problem was no 10MM socket or offset wrench. I picked up a 10mm socket at Ace Hardware but then noticed it is too short with my 1/4" ratchet handle  to reach over the outlet of the housing. Pressing on, I removed the hose and was able to reach the lower bolt reaching under the open end of the outlet.  After removing the bolts I see that they have torx socket as well as the 10mm hex head. I have the appropriate 1/4" drive  torx socket but it is too short to reach over the outlet and I don't have a short 1/4" drive extension. 

 

As I removed the housing from the engine, a good quantity of coolant spilled out of the engine while  none appeared to continue to leak from the outlet after the initial amount when removing the water hose. I expected to find the thermostat in the housing stuck open but it looks good by eye - no gap visible between the seat and valve plate if stuck open.  If it were stuck open, I'd expected to see all available coolant to have leaked out the outlet after the hose was removed with none remaining to leak from the engine. 

 

I tested the thermostat/housing  by holding it under a tap and filling the outlet expecting to see water leak out quickly after moving it out of the water stream. Some water leaked out but in never emptied. I did the same test with the new DORMAN and surprise surprise there is a small hole in the valve plate that allows the water to drain quicker then the suspect failed thermostat (no similar hole in the OE thermostat?!?).  At that point I was alarmed but committed to replacing the old suspect thermostat as there is no way I would re-install the 12 year old thermostat.  (I put it in a pan of water and brought the temperature up slowly and can see that it is open at 190 Deg F, and quickly moving it under a stream of water see that the water drain freely around the open valve plate until the temperature dropped and water free flow stopped.)

 

After the installation was complete, I added 50/50 Dexcool to the reservoir tank, pored fresh water over the belts and pulleys below the housing to flush off the spilled coolent, started it up and allowed it to run at idle. Temperature rose quickly to over 190 and reached something over 200.  Since I did not take pictures to record the values this is reported from memory. Temperature reached the point the radiator fan turned on. 

 

The next morning  6 Deg F ambient, I used the remote start to warm it up.  We drove off before the remote start timed out to shut down, temperature was indicated at 170 Deg F. The car warmed up nicely and reached 195 Deg for most of the drive and reached 202 stopped at traffic lights.  Problem solved.

[John/Horace]

Was that the factory original HOAT coolant in the engine still ? Early 2013 vehicles had hoat which is not compatible with oat, the coolant still used today.

Hoat is purplish when new, oat is closer to an orange color; kinda tricky for reliable assessment.

 

Dexcool does make oat coolant, compatible with other orange oat extended life coolants. But mixing hoat and oat coolant can make jell over time that will probably plug up your dash heater core etc.

 

To avoid any problems I would dump out all coolant and flush system with distilled water if possible. Then refill with a oat (organic acid technology) long life fluid; which will last vehicle life time. Google what I saying, this potential problem is out there. Heater core is not easy to change, even rad can have issues.

 

I'm in my mid 60s, working on cars is not as easy as it was years ago. But information is more available. Cheers.

[5rebel9]

Well, you did well for your experience. I have stopped bothering doing the very  "old school" method of checking the stat in hot water, never seemed to help be sure of things and took time away that could have been better spent.

 And yes our 2  3.6 motored DJ's run those same basic temps in this kind of weather.

Those 2 stat bolts are a small inconvenience to get at and I've done stats that are a LOT harder to get at!

Glad to hear all is good now for you and your DJ!