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Discussion Starter #1
Hello all. I know there are more than a few P0128 code threads out there but after scanning many of them I haven't found an answer that works for my TJ yet. It's a 2006 TJ 2.4L. Here is what I've done to try to rectify the problem:
1. Flushed and Burped coolant (on incline) and checked levels.
2. Replaced Thermostat with Stant (45209) SuperStat 195 Degree, got code again so tried Stant (14209) 195 Degree.
3. Replaced Coolant Temperature Sensor
4. After reading this article https://www.obd-codes.com/p0128 I replaced the Intake Air Temp sensor.

Heat works as it should. Code has tripped in hot outside temps as well as cool temps. Any more things to try? If I decide to seek professional help, would it better to take it to a dealer or let a trusted local mechanic take a look?

Thank you all for your time and input
 

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What does your temp gauge read when fully warmed up? About 210 is the normal reading.



Is your electric fan running all of the time or cycling as it normally should (A/C off)?
 

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Discussion Starter #3
The fan cycles as it should. I hooked up my OBD to it and checked it every 5 min at idle. This is what I found:
Time - ECT (in degrees F)
5 min - 135
10 min - 154
15 min - 169
20 min - 192
25 min - 212
 

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At what point in time during your 5 min. checks did the fan come on?

Taking more than 20 min. to get up to operating temp seems a little long & would normally indicate the thermostat in opening too soon, like if you have a stuck open thermostat or possibly a lower temp rated thermostat even though you have had 2 different 195° ones.

On my ’05, 4.0 even on cool days it will usually get up to 210° within a mile or two which would probably be 5 minutes or so. Of course that is well above idle but even at idle that seems long.
 

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Discussion Starter #5
I wasn’t staying with the Jeep the whole time but the first I noticed it coming on was at the 20 min mark. I agree that things are pointing to a stuck open thermostat. I may try buying the oem Mopar one and seeing if that helps. And then opening a used thermostat store 🙂
 

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Discussion Starter #6
So after installing the Mopar thermostat I again got the code. I hate throwing in the towel but it’s got me stumped. Do you all think the dealer would be the best place to get it looked at or a local mechanic?
 

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I can’t think of anything other than the thermostat that would cause longer than normal time to come up to operating temperature.

When you are driving, how long does it actually take from cold start up to get to 210° on the gauge? And at what point (on the gauge) does the CEL come on? Before it gets to 210°?



The problem with taking a 13 year old vehicle to a dealer is they probably have not had one that old in for more that maybe routine maintenance in maybe 10 years. And there may not be any techs in the shop that have even worked there for 10 years. So getting someone with any real world experience with your’06 is probably remote, at best. The dealers often have a habit when they don’t know a cause say the PCM is bad. Most of the time they are wrong & in your case new PCMs are not even available so even if correct they really can’t get a reliably good replacement one. *


You may find that an independent shop will lack vehicle specific experience too.

Dealing with an uncommon problem as you apparently have complicates things further. I would say call around explaining the problem & what you have done & see what they have to say. Unfortunately most of the time they just say bring it in & we will check it.



* Here is an example of someone having this issue with the dealer saying he needs a PCM & what they told him.

https://www.wranglerforum.com/f210/2005-wrangler-jeep-says-computer-is-fried-but-they-cannot-provide-a-new-one-2375495.html
 

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Discussion Starter #8
Thanks for the reply Rubi. It took close to 20 minutes driving time for the gauge to come up to 200 degrees. Probably about 12-15 to get up past 190. We had a cold night last night and I noticed that the heat still works pretty quickly and gets nice and hot.
The light did seem to come on during the same driving condition a couple times but completely random other times. I guess I can try to pinpoint when the light comes on; RPM’s, time driven, temp, etc.
 

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That IS way to long to reach operating temp, assuming the gauge is correct. The PCM is using that gauge data to determine the engine temp & when/if to set the code.

The heater gets its coolant from “below” the thermostat (as does the sensor) but what “feels” hot is not necessarily the best test but happening relatively soon does indicate the thermostat is likely closed, at least initially.

The only thing left to do is to see it the actual temp is the same as the gauge. If you do not have an infrared thermometer (less than $20 at places like Harbor Freight) you can try starting it cold & waiting until the top radiator hose starts getting hot to the touch indicating the thermostat is opening allowing hot engine coolant into the radiator. Then checking the gauge. By that time or certainly when you can’t hold you hand on the hose the temp & therefore the gauge should be at or close to the 210° mark.

 

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Discussion Starter #10
Thanks again Rubi. I ordered an infrared thermometer and will check that when it arrives. I see that the temp gauge works when the coolant temp sensor is unplugged, but my OBD does not. Could the issue be with the gauge or does the computer work off the CTS? Just trying different ideas.
 

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One problem if you try to use an app or live data scanner to read the temp (from the data port) you will be using the same “interpreted” data that the gauge is using. That is why you need to get the temp directly from the radiator. Be sure to read it from the top, right at the top radiator hose for most accurate reading compared to the temp at the thermostat/sensor area.

Adding to the uncertainty is the fact that the PCM, presumably using sensor data, is what sets the codes so even if the temps are as high & as soon as they should be but the PCM is not seeing it, you still have the code popping up…


I was reading an interesting thread on the accuracy of the oil pressure gauge. Not very accurate compared to a mechanical gauge. Not too promising for the temp accuracy…

https://www.jeepforum.com/forum/f9/97-tj-4-0-oil-pressure-sender-bench-test-1389952/
 

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Discussion Starter #12
That is an interesting read. Jeeps can be as frustrating as they are fun.

I did notice after a short ride today that the temp read 176 but the top radiator hose was very hot. I take this as meaning the thermostat was opened too soon (which I doubt) or that the engine is up to temp as it should be but not “sensing” it. I should have the infrared thermometer tomorrow or Tuesday and post what I find. Thanks again!
 

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Discussion Starter #13
So after using the infrared thermometer on the top hose I noticed that when the hose read 210 the ECT was showing around 190. However as I moved the Gun toward the hose where it turns down into the engine and thermostat housing, it was more equal to what the ECT showed. I know you said to use it at the top near radiator, I was just adding what I found.
Also, as the engine was warming up the ECT was at a higher temp than the top hose but I assume that’s because the thermostat hadn’t opened yet?
The hose started to surpass the ECT in temperature around 175 degrees.

Does the comp read from the ECT or the dashboard gauge? Because the dash gauge still functions when the ECT is disconnected.
 

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Radiator, hose or housing position for reading is fine & you got the expected results (closest to the hot coolant under the thermostat, the higher the readings). I just did not want you reading it too far from that source which would affect the temps, lower.

I’m not sure why the gauge stays up when the sensor is disconnected. If you turn the key off & back on does the gauge go down when key is off? And does it stay down when the key is back on or does it return to previous reading?

I understand that the PCM is able to use other inputs to approximate the coolant temp in certain situations so that may or may not be in play here. If that is what is happening I would guess that if you disconnect at, say 160° (gauge) & leave it running the gauge would continue on up over time while unplugged. That might test that theory.

The PCM definitely reads the sensor then sends the data over the PCI bus to the cluster as well as the data port.

As for your results basically the infrared & sensor (gauge) readings are fairly similar as the temp rises from cold to normal operating temp. If so I guess that says it is actually taking that (excessively long) time to get up to temp. Right??
 
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