Engine Temperature Keeps Getting Lost and Fans Run at Full Speed

[L8T BURB]

A few months ago, just as I was getting home, the engine fan started going full speed and I could hear it over my radio. I also noticed that the A/C stopped working and even after I got home and turned the engine off, the fan stayed on for 5 minutes on my 2015 Chevy Tahoe. I had a friend who was a mechanic come and check the truck and saw that there was no temperature reading and explained to me that since the computer could not tell what the temp of the engine was, it went into failsafe mode and engaged the fan at full speed. He told me that it was most likely the sensor and suggested I replace it. Two days later, before I could buy the replacement, the problem went away, but then came back 3 weeks later. The problem went away the very next day and the truck was fine for a week or so, then the problem came back so I had the sensor replaced hoping that this loss of engine reading would go away. A week later, the problem came back and then it went away and this has been going for months. The problem seems to have more and more often so I have been tracking this issue for the last four weeks and I can tell you the following for certain:

- The problem starts after 10-15 minutes of going on the freeway and the problem persists until I shut down the engine for a few hours, and then go back on the freeway.

This has been driving me insane because I have driven the truck for days in the city and the problem persists until I go to work and have to be on the freeway driving at 65 miles an hour. Once the problem goes away, it will not come back until I get on the freeway again. It doesn't always happen but for the past 6 days, it has been very consistent. I drive to work and before I get there, the fan is going full speed and I get no temp reading. Once I am on my way home, by the time I get off the freeway, the temperature reading is back and the engine works normally. I want to say it is probably a short somewhere but this consistent behavior is throwing me off. I have been hesitant to take the truck to the mechanic because I feel they are going to do some work, the problem will go away because somehow the problem gets resolved and then the problem comes back because what they did was not really the problem so I am hoping someone has had this problem and has a better insight. If I do ever find out what is wrong with mine, I will make sure to share it.
Thank you for reading.

I'll bet the house that a scanner will show you "coolant temperature below threshold". It will not illuminate the CEL, but even a cheap scanner from Harbor Freight will detect the stored code.

Rather than write a novel explaining my position on this, I'll save you the time of extensive reading and headache, and likely some money as well.

Replace the thermostat with a GM genuine thermostat. The coolant temp sensor is NOT the culprit. People get confused thinking it is, only to end up replacing it multiple times over and over before realizing the thermostat was the issue all along. Never once have I heard of someone replacing a thermostat with a GM genuine one, more than once.

Once you replace the thermostat, again... must use GM Genuine part, top off the coolant and clear any codes with cheap scanner. Guaranteed this will never happen again. I have gone through this issue myself, so I am speaking from experience and not theory. If you're unable or unwilling to source a scanner tool, removing the negative battery cable for a period of time may clear the code.

Please report back with your findings.

 

[K2 Kaiju]

LOL, mine did this last night. This is your answer:

 

[Doubeleive]

I'll bet the house that a scanner will show you "coolant temperature below threshold". It will not illuminate the CEL, but even a cheap scanner from Harbor Freight will detect the stored code.

Rather than write a novel explaining my position on this, I'll save you the time of extensive reading and headache, and likely some money as well.

Replace the thermostat with a GM genuine thermostat. The coolant temp sensor is NOT the culprit. People get confused thinking it is, only to end up replacing it multiple times over and over before realizing the thermostat was the issue all along. Never once have I heard of someone replacing a thermostat with a GM genuine one, more than once.

Once you replace the thermostat, again... must use GM Genuine part, top off the coolant and clear any codes with cheap scanner. Guaranteed this will never happen again. I have gone through this issue myself, so I am speaking from experience and not theory. If you're unable or unwilling to source a scanner tool, removing the negative battery cable for a period of time may clear the code.

Please report back with your findings.

either one can go bad or both

 

[Boston]

We need the underlying trouble codes that are generating this warning light on your dashboard.

I recommend that you get yourself a quality bi-directional scanner to add to your tool box. Then, you will be able to read ALL codes, along with a brief explanation/pointer to the source of your issue. You will literally save thousands of dollars in diagnostic and repair costs over the life of your truck, and you will most likely recoup the cost of the scanner within 1 year.

Ditto!

 

[Krbobby]

try the above suggestions but know that doing things like unplugging it and or disconnecting the battery sort of "resets" it for a few days and it may continue repeating the same issue.
if that happens source a new OEM temperature sensor from the dealer or from a local auto parts.
there is a bad rash of defective temp sensors and thermostats out there, this comes up ALL THE TIME now on here last post was just a week or so ago and a few days before that and a few day before that and so on, it has become a very common issue
some people have had to replace the sensor multiple times, as to if those are oem or not I do not know but I suspect not.

Once I replaced the thermostat a few weeks ago, we’ve had zero issues since and the heat works much better.

 

[Doubeleive]

Once I replaced the thermostat a few weeks ago, we’ve had zero issues since and the heat works much better.

ya the thermostat being defective seems to be the most common fix, sure seems to happen more than it should though. you don't see this happening on previous generations, pretty rare.

 

[jaxscuby]

so for those that bought GM thermostat, 190 degrees?

 

[K2 Kaiju]

so for those that bought GM thermostat, 190 degrees?

https://a.co/d/5Gn2bzR I got this one plus the dorman housing and AC delco juice. Works perfect now.

 

[BartAAllen]

Hello, I have read through all replies in this thread and have a simple question...Why would the temperature gauge fail to display a temperature (needle goes all the way to the left) instantly if the thermostat was bad??? I would not think the gauge would do this unless it wasn't getting a signal. I also wouldn't expect that the thermostat would have any affect on the gauge just instantaneously failing. Thus this is why I would expect the temperature sensor is the problem or its connector. Please let me know if someone has an explanation on how the thermostat could caus a gauge reading to instantly go to far left. I am having the same issues as discussed here and have only replaced the coolant sensor 3 times but not with genuine GM sensor (which could be the problem). Or it could be the connector to the sensor. Your thoughts would be appreciated!

 

[L8T BURB]

Hello, I have read through all replies in this thread and have a simple question...Why would the temperature gauge fail to display a temperature (needle goes all the way to the left) instantly if the thermostat was bad??? I would not think the gauge would do this unless it wasn't getting a signal. I also wouldn't expect that the thermostat would have any affect on the gauge just instantaneously failing. Thus this is why I would expect the temperature sensor is the problem or its connector. Please let me know if someone has an explanation on how the thermostat could caus a gauge reading to instantly go to far left. I am having the same issues as discussed here and have only replaced the coolant sensor 3 times but not with genuine GM sensor (which could be the problem). Or it could be the connector to the sensor. Your thoughts would be appreciated!

The fact that you have replaced the coolant temp sensor 3 times should tell you that you're on the wrong track. No worries though, most members here with this issue have done the same thing (myself included).

The issue is definitely with the thermostat and nothing to do with the coolant temp sensor. I had this precise issue with my 2015 Suburban. Replaced the coolant temperature sensor twice, problem would clear up a couple days each time before returning again.

Finally I replaced the thermostat with a GM genuine part and problem totally solved. My theory is that the thermostat (both on 5.3 and 6.2 applications) becomes faulty and stays partially opened at all times, not allowing the coolant to reach full operating temps of 210. This seems to be an intermittent issue when the weather is very warm outside, but when ambient temps are below 50 degrees, there's no chance that a faulty thermostat thats partially stuck open will get you to 210. When the engine runs for an extended period and cannot get to 210, it will eventually go to "fail safe" mode, engaging the cooling fans non-stop, sometimes including a few minutes after shut-off, not allowing the AC compressor to engage, and the coolant temp gauge will drop to zero. This fail safe mode is prompted by a stored code of "coolant temp below threshold". Don't believe it, scan your truck for codes when the temp gauge is zero and you'll see. A check engine light doesn't need to be on to read stored codes.

I have seen posts on this forum where people immediately start talking about wiring harnesses, ECM replacements, broken wires, etc. Such costly repairs that do not solve the issue, but there's such little information out there with definitive results, which has made this issue so elusive to many. What should be a ~$100 repair, ends up being several hundred or thousands while the faulty thermostat was truly the root cause of it all.

The temperature sensor (or the wires connected to it) are never the issue. Replace the thermostat with a Genuine GM one and reset the stored engine code and you'll be good to go. Before you say it, "but I don't have an engine light"... when the fans start running like that and temp drops to zero, at that moment, the ECM stores a code of "coolant temperature below threshold".

If you have your original temp sensor, throw it back on there just to show the members here that it never was a temp sensor issue. While I understand both the temp sensor and thermostat can fail at some point, the root cause is the thermostat. People get confused because of the temp gauge dropping to zero and automatically want to point to the temp sensor. Just search the forum... I haven't noticed a single person with these symptoms who replaced thermostat and had continued issues. However those that have replaced the coolant temperature sensor have... well... you get my point lol.

You've got a stored code for "coolant temp below threshold". This code is what causes the temp gauge to drop to 0 and the other fail safe modes to activate. You will need to get that code cleared after thermostat replacement before things will fully resume normal operations. If you don't have a pocket scanner or similar to clear the code yourself, I believe you can disconnect the battery for 30 minutes or so and it may clear it.