[SOLVED] 2012 6.2L Yukon Denali XL 172000 miles? Need help and cannot figure this very random problem out.

testdepth · Feb 22, 2026

I've tossed in the towel and took it to my local shop who's owner I've known for years. He says his diagnostic guy is very good. This truck sat all night and all day today until 630pm when I got in to drive it over and drop it off. Happened again with the same scenario: Get in and starts right up and idles normally. Engine temp is bottomed out at 160F. I drove down the street to the stop sign and then to the traffic light with no problems. I turned right and accelerated down to the next traffic light and stopped with no problem. Sitting there in drive but stopped and idling no problem. I turn left and drive a half a block (engine temp about 190F) when the truck starts surging and bucking with RPMs jumping and acting like its running out of gas but it did not stall this time. This happened at about 35mph. It finally caught itself and by then was at normal temp so it ran fine. Hopefully his diagnostic tech will sit there tomorrow looking at live data while one of the other techs drives and it will have this problem and he can tell me why it's doing this.

@Charlie207 @Doubeleive @rdezs

rdezs · Feb 23, 2026

That's interesting regarding engine temperature. Makes me wonder about your engine coolant temperature sending unit (on the driver side cylinder head in front of the number one spark plug), or the ambient Air temperature sensor. Maybe the ECM thinks it's -25° outside and is altering fuel mixture and timing. To be able to see all that on a good bi-directional scanner or a Tech 2.

testdepth · Feb 23, 2026

@rdezs @Charlie207 @Doubeleive

It's been at the shop all day and I just spoke to them. Thankfully they were able to recreate the problem while one guy drove and their diagnostic guy held the scan tool. While the problem was happening the coolant temp sensor showed -41 degrees. This is a brand new sensor. This guy started fiddling with the wiring to the coolant temp sensor and the temperature came back to normal while he watched the scan tool. He repaired the wiring. That sensor is located right next to the driver side exhaust manifold. Heatshields everywhere else on this truck but not on the coolant sensor. Thanks GM engineer idiots. I told the guy to keep it over night and try again in the morning to verify it's fixed. This was the only issue he found with the entire truck. I'll report back tomorrow.

Doubeleive · Feb 23, 2026

testdepth said:
@rdezs @Charlie207 @Doubeleive

It's been at the shop all day and I just spoke to them. Thankfully they were able to recreate the problem while one guy drove and their diagnostic guy held the scan tool. While the problem was happening the coolant temp sensor showed -41 degrees. This is a brand new sensor. This guy started fiddling with the wiring to the coolant temp sensor and the temperature came back to normal while he watched the scan tool. He repaired the wiring. That sensor is located right next to the driver side exhaust manifold. Heatshields everywhere else on this truck but not on the coolant sensor. Thanks GM engineer idiots. I told the guy to keep it over night and try again in the morning to verify it's fixed. This was the only issue he found with the entire truck. I'll report back tomorrow.

that can do it, a scanner would have revealed it.... if you looked for it
it wouldn't likely throw a code but it would cause a problem between actual temp and what it thinks it is (what the sensor shows)

rdezs · Feb 23, 2026

rdezs said:
That's interesting regarding engine temperature. Makes me wonder about your engine coolant temperature sending unit (on the driver side cylinder head in front of the number one spark plug), or the ambient Air temperature sensor. Maybe the ECM thinks it's -25° outside and is altering fuel mixture and timing. To be able to see all that on a good bi-directional scanner or a Tech 2.

Actually a pretty common issue. The computer depends on knowing the correct engine temperature as well as the ambient

testdepth · Feb 23, 2026

I hate newer cars with computers. So much simpler with a TH350 and a SBC.

SpareParts · Feb 23, 2026

I have to do it. Post #3 was right, i think.

Doubeleive · Feb 23, 2026

SpareParts said:
I have to do it. Post #3 was right, i think.

a lot of coulda shoulda I told him to look at the data.
-40 makes it dump fuel "super cold start" that is it the very bottom of the sensors range, then the maf see's a more accurate ambient temp and I would imagine this causes a messed air-fuel mixture.
kind of like leaving the choke open on you weed whacker after it has warmed up, it just dies when you give it throttle

testdepth · Feb 24, 2026

Well they drove it this morning and called to tell me it's fixed and come get it. They replaced the connector to the coolant temperature sensor. They found no other problems with this Yukon.

Thank you very much for all of your inputs and guidance!

Charlie207 · Feb 25, 2026

testdepth said:
Well they drove it this morning and called to tell me it's fixed and come get it. They replaced the connector to the coolant temperature sensor. They found no other problems with this Yukon.

Thank you very much for all of your inputs and guidance!

Glad to hear!

rdezs · Feb 26, 2026

I use a heat resistant sleeve over the coolant sending unit wire, just large enough to cover the plastic connector itself.

That thing gets baked from the manifold over time, and it's pretty common to have the plastic connector crumble between your fingers when unplugging it.

The 2014 Escalade actually has a heat protection sleeve over the wiring, but not over the plastic plug.... My 2003 Hummer with a 6.0 had nothing but bare wires leading to it, so it's sometime in the past GM made an attempt to protect it.

Charlie207 · Feb 26, 2026

rdezs said:
I use a heat resistant sleeve over the coolant sending unit wire, just large enough to cover the plastic connector itself.

That thing gets baked from the manifold over time, and it's pretty common to have the plastic connector crumble between your fingers when unplugging it.

The 2014 Escalade actually has a heat protection sleeve over the wiring, but not over the plastic plug.... My 2003 Hummer with a 6.0 had nothing but bare wires leading to it, so it's sometime in the past GM made an attempt to protect it.

Does the coolant temp sensor harness plug into the loom with another harness (eg: can I buy a new one?), or does it splice into the larger loom, and require cutting/splicing to replace? I don't mind doing either (it's just two-wire?) but I'd rather just unclip/clip and then wrap in heat shielding.

rdezs · Feb 26, 2026

The new plug comes with about 8 in of wire that you need to splice, solder and heat shrink. I use the fiberglass heat resistant tubing.... Compress it together and it gets larger, pull it lengthwise and it gets smaller. A stainless steel metal zip tie like what you see on CV and driveline boots, but smaller, works great.

Tim907356 · May 18, 2026

I hope you don't mind a piggyback, but I just had a similar issue, and hope those engaged on your thread can help. 2010 Yukon Denali XL 6.2l Flex. Driving over the weekend, a message came up: Engine Temperature Hot; A/C turned off. Sure enough, no A/C, but also the engine temperature gauge was showing 160 deg. - needle pegged. We continued, with the truck driving fine otherwise. I got there and wiggled the temperature sensor wires to no avail. I pulled the battery cable for 5min., re-attached it, and turned the key, and the gauge came back to life. We made a few short trips around our destination, and on about the 3rd. trip, the needle drops to 160F, and I get the same message. I repeated the same wire-wiggle, and battery cycle, and it restored the gauge function, and thankfully the A/C (93F in SC at the moment) for the 4hr. ride home. So, right now that's the solution. I guess the battery reset gives me a couple of starts before it's going to crap out again, but its a weird scenario if its a sensor issue. I plan to replace the sensor today in any event. I should mention, I just put a new aftermarket radiator in, due to overheating while towing a camper in the summer, covered on another thread.
TIA!

j91z28d1 · May 18, 2026

Tim907356 said:
I hope you don't mind a piggyback, but I just had a similar issue, and hope those engaged on your thread can help. 2010 Yukon Denali XL 6.2l Flex. Driving over the weekend, a message came up: Engine Temperature Hot; A/C turned off. Sure enough, no A/C, but also the engine temperature gauge was showing 160 deg. - needle pegged. We continued, with the truck driving fine otherwise. I got there and wiggled the temperature sensor wires to no avail. I pulled the battery cable for 5min., re-attached it, and turned the key, and the gauge came back to life. We made a few short trips around our destination, and on about the 3rd. trip, the needle drops to 160F, and I get the same message. I repeated the same wire-wiggle, and battery cycle, and it restored the gauge function, and thankfully the A/C (93F in SC at the moment) for the 4hr. ride home. So, right now that's the solution. I guess the battery reset gives me a couple of starts before it's going to crap out again, but its a weird scenario if its a sensor issue. I plan to replace the sensor today in any event. I should mention, I just put a new aftermarket radiator in, due to overheating while towing a camper in the summer, covered on another thread.
TIA!

try replacing both batteries cables, ground and pos. it's a common issue for these trucks. for some reason they look perfect from the outside, so people are reluctant to replace the them. but these trucks hate voltage issues.

Foggy · May 18, 2026

Tim907356 said:
I hope you don't mind a piggyback, but I just had a similar issue, and hope those engaged on your thread can help. 2010 Yukon Denali XL 6.2l Flex. Driving over the weekend, a message came up: Engine Temperature Hot; A/C turned off. Sure enough, no A/C, but also the engine temperature gauge was showing 160 deg. - needle pegged. We continued, with the truck driving fine otherwise. I got there and wiggled the temperature sensor wires to no avail. I pulled the battery cable for 5min., re-attached it, and turned the key, and the gauge came back to life. We made a few short trips around our destination, and on about the 3rd. trip, the needle drops to 160F, and I get the same message. I repeated the same wire-wiggle, and battery cycle, and it restored the gauge function, and thankfully the A/C (93F in SC at the moment) for the 4hr. ride home. So, right now that's the solution. I guess the battery reset gives me a couple of starts before it's going to crap out again, but its aProb weird scenario if its a sensor issue. I plan to replace the sensor today in any event. I should mention, I just put a new aftermarket radiator in, due to overheating while towing a camper in the summer, covered on another thread.
TIA!

Probably a good idea to replace the thermostat as well. A lot of temp codes
can show up with a goofy thermostat.. Just not overheating, but "underheating"

rdezs · May 18, 2026

If you really want to get in the weeds on engine temperature.... On LS engines the driver side cylinder head runs warmer, usually about 10°. It has a longer path to the thermostat, also the passenger side get some flow from the heater core lines. With the coolant temperature sending unit measuring the hottest spot in the driver side cylinder head.... Right where the coolant is exiting and right next to an exhaust port.... The indicated temperature isn't really accurate.

Considering you're not directly measuring the temperature.... It's interpreted by some algorithm in the software in the ECM and translated to your gauge.... It's basically a ballpark figure measured at the hottest point in the engine. You can either pull out an infrared heat gun and check your actual temperatures around both cylinder heads, or install an aftermarket sending unit in the passenger head which will show about 10° cooler.

So.... If your gauge reads 230°.. I wouldn't worry about that too much.

kbuskill · May 20, 2026

Tim907356 said:
I hope you don't mind a piggyback, but I just had a similar issue, and hope those engaged on your thread can help. 2010 Yukon Denali XL 6.2l Flex. Driving over the weekend, a message came up: Engine Temperature Hot; A/C turned off. Sure enough, no A/C, but also the engine temperature gauge was showing 160 deg. - needle pegged. We continued, with the truck driving fine otherwise. I got there and wiggled the temperature sensor wires to no avail. I pulled the battery cable for 5min., re-attached it, and turned the key, and the gauge came back to life. We made a few short trips around our destination, and on about the 3rd. trip, the needle drops to 160F, and I get the same message. I repeated the same wire-wiggle, and battery cycle, and it restored the gauge function, and thankfully the A/C (93F in SC at the moment) for the 4hr. ride home. So, right now that's the solution. I guess the battery reset gives me a couple of starts before it's going to crap out again, but its a weird scenario if its a sensor issue. I plan to replace the sensor today in any event. I should mention, I just put a new aftermarket radiator in, due to overheating while towing a camper in the summer, covered on another thread.
TIA!

I had the same issue when I installed a Cold Case all aluminum radiator in my Suburban. In my case the radiator doesn't have a restrictor in the port where the small hose attaches to the passenger side end tank and goes back to the thermostat housing. This allows to much water to by pass the thermostat and keeps the truck from getting up to operating condition quickly. GM has these trucks programmed to look for a certain amount of temperature rise within a certain time frame of driving. If it doesn't get up to temperature fast enough the ECU thinks there must not be water in the system to cause the sensor to read correctly and will put the truck into a limp mode where it turns off the A/C and will turn the radiator fans on High speed to try to cool the engine down, even though it actually isn't overheating.

I made a restrictor from an old metal TPMS sensor valve stem and put it into that port on the radiator and now the truck runs beautifully.

[SOLVED] 2012 6.2L Yukon Denali XL 172000 miles? Need help and cannot figure this very random problem out.

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