Now at FIVE AC system failures in four weeks...

[Lonny]

So, the abundance of crud in the system is still trapped in the evaporator. Bits and pieces and chunks are working loose and going on vacation inside the system. The evaporator is not absorbing heat efficiently.

Do you think I should bring it to a GMC dealer at this point? I’m a little worried about the prospect of starting the diagnostic cycle (and OOP) costs all over again…

Or just sell the thing? CarMax has a few of the same model year with more miles for the EXACT same price I bought mine five years ago…

 

[L8T BURB]

This is likely a HUGE shot in the dark, but figured it may be worth mentioning.

Does your engine temperature gauge get to, and stay right at 210 during normal operations? I am asking because the thermostat on these engines are very weak and known to fail. They get stuck partially open, allowing engine temp to run below what it is designed. If operated long enough under these circumstances, it causes a fail safe mode to activate where the AC compressor will no longer engage, and cooling fans will run on high speed non-stop. This can occasionally reset before the next drive cycle, causing this to only be intermittent which is even more confusing to diagnose.

Perhaps a couple things to check to see if this could be something to consider....

1. Does the AC compressor engage when the vents are blowing the "not cold" air?
2. Does your engine temp gauge point straight up at 210 after 5-10 minutes of operation?
3. When the AC is acting up does the engine cooling fans run on high non-stop?

If the answer is yes to these, to 1 and 2, but no to 3, my theory is fully defeated. If the answer is no to 1, 2, and/or yes to 3, I would recommend throwing a GM thermostat and GM coolant temperature sensor on it for good measure.

Hope this helps!

 

[OR VietVet]

Do you think I should bring it to a GMC dealer at this point? I’m a little worried about the prospect of starting the diagnostic cycle (and OOP) costs all over again…

Or just sell the thing? CarMax has a few of the same model year with more miles for the EXACT same price I bought mine five years ago…

I would at least let a sealer or a trusted other shop look at it, after you lay out the whole story in chronological order.

 

[Lonny]

This is likely a HUGE shot in the dark, but figured it may be worth mentioning.

Does your engine temperature gauge get to, and stay right at 210 during normal operations? I am asking because the thermostat on these engines are very weak and known to fail. They get stuck partially open, allowing engine temp to run below what it is designed. If operated long enough under these circumstances, it causes a fail safe mode to activate where the AC compressor will no longer engage, and cooling fans will run on high speed non-stop. This can occasionally reset before the next drive cycle, causing this to only be intermittent which is even more confusing to diagnose.

Perhaps a couple things to check to see if this could be something to consider....

1. Does the AC compressor engage when the vents are blowing the "not cold" air?
2. Does your engine temp gauge point straight up at 210 after 5-10 minutes of operation?
3. When the AC is acting up does the engine cooling fans run on high non-stop?

If the answer is yes to these, to 1 and 2, but no to 3, my theory is fully defeated. If the answer is no to 1, 2, and/or yes to 3, I would recommend throwing a GM thermostat and GM coolant temperature sensor on it for good measure.

Hope this helps!

Thanks for all of this. I took the truck out again early tonight for a half hour and here’s what I found:

1. Yes, it does seem that the compressor engages.

2. Nope! The engine temp holds steady at 160, right down the middle.

3. Yes. It does seem that the fans run continuously.

4. I also noticed that 68 degrees appeared to be the temp setting where the AC maxed out in terms of its cooling capability.

 

[swathdiver]

So what brand of parts are they using? Does not seem like GM OE parts or even ACDelco Professional/Gold.

 

[Lonny]

So what brand of parts are they using? Does not seem like GM OE parts or even ACDelco Professional/Gold.

I posed this very question to the service guys after the very first go-round. He assured me that everything going into the truck was either GM or ACDelco. I verified what he was telling me was accurate on the service records after that first repair.

And the second.
And the third.
And the fourth.
(And here we are).

In happier news, I am pleased to report that I had the opportunity to drive a bunch of different rental cars, from a Chevy Impala (noisy, uncomfortable, terrible), Ford Mustang Mach E (sporty fun little thing, but small!), and some sort of Cadillac Micro-SUV that looked and felt more like a Rav-4. Or a toaster oven.

Today we drive back to San Diego from Palm Springs. This may be the tipping point where I want the thing fixed enough to sell.

 

[L8T BURB]

Thanks for all of this. I took the truck out again early tonight for a half hour and here’s what I found:

1. Yes, it does seem that the compressor engages.

2. Nope! The engine temp holds steady at 160, right down the middle.

3. Yes. It does seem that the fans run continuously.

4. I also noticed that 68 degrees appeared to be the temp setting where the AC maxed out in terms of its cooling capability.

Due to your responses here, I would HIGHLY recommend a GM thermostat and GM coolant temperature sensor. Around $100 in parts, but I think these symptoms warrant this repair. Trying to justify to a dealer or warranty company why this repair should happen may be challenging, but due to the relatively low cost and potential of it to get you closer to a resolution, I think it's worth doing it.

 

[Lonny]

Due to your responses here, I would HIGHLY recommend a GM thermostat and GM coolant temperature sensor. Around $100 in parts, but I think these symptoms warrant this repair. Trying to justify to a dealer or warranty company why this repair should happen may be challenging, but due to the relatively low cost and potential of it to get you closer to a resolution, I think it's worth doing it.

Completely agree with everything you said! I have this on my list to check when I get home to see if the sensors were replaced during any one of the previous four attempts.

Reminds me of a problem I had last year with our swimming pool not heating up...a $30K rooftop solar system rendered inactive because of a faulty $8 thermostat I was able to buy on Amazon and install on my own.

 

[swathdiver]

I posed this very question to the service guys after the very first go-round. He assured me that everything going into the truck was either GM or ACDelco. I verified what he was telling me was accurate on the service records after that first repair.

And the second.
And the third.
And the fourth.
(And here we are).

In happier news, I am pleased to report that I had the opportunity to drive a bunch of different rental cars, from a Chevy Impala (noisy, uncomfortable, terrible), Ford Mustang Mach E (sporty fun little thing, but small!), and some sort of Cadillac Micro-SUV that looked and felt more like a Rav-4. Or a toaster oven.

Today we drive back to San Diego from Palm Springs. This may be the tipping point where I want the thing fixed enough to sell.

I'm weary, GM parts usually don't fail like that. My daughter had her girlfriend's mechanic boyfriend replace her cooling fan once. Upon installing the 3rd fan motor I was asked to come outside and have a look. The kid was shorting out each new fan by wiring them up wrong and of course, blaming the parts as being defective. I told him how to wire up the 4th one and that was that.

 

[Lonny]

Ok guys, today is the day...after a weekend of intermittent performance on the AC system, I'm now back in San Diego and have a service appointment at my local GMC dealership.

I'll be bringing with me the service records of the previous four shop visits and hopefully we'll get some indication what is going on.

On a related note, my extended warranty expires in 2026 on my current Denali and so it looks like I'll be making the move into something new (or newer) next year. I know a Denali refresh is coming in a couple months, which is intriguing...but then, at that cost we're nearly into Escalade territory. The dealership I'm going to today is Cadillac/GMC so I'll get a chance to do a bit of an initial cross-shop. Would love any input if you guys have cross-shopped yourselves between the GMC and Cadillac.

 

[Lonny]

Back at the house now with yet another rental car. Unfortunately my "appointment" at the GMC/Cadillac dealership was nothing more than an opportunity to drop off the Denali after speaking with someone from the service desk for five minutes to explain the problem. Apparently it's going to take them at least a day or two before they can even look at the thing.

Interestingly, the service manager did mention to me that there are certain components associated with the AC system that require the entire front end to be removed and taken apart - a job that take days and will cost something like $5,000. Fortunately my warranty will cover it, but it could explain why the previous service guys were apprehensive about digging into the problem further than a condenser or compressor.

 

[OR VietVet]

The condenser is up front and to my knowledge, that does not require "entire front end removal". Maybe someone with info on newer rigs can chime in.

 

[Lonny]

Hi team -

Just heard back from the GMC dealership with the diagnosis: they discovered that the connector hoses are leaking. It's a $3,000 job that will fortunately be covered by warranty. I should have some more information later from them today or tomorrow.

 

[swathdiver]

The condenser is up front and to my knowledge, that does not require "entire front end removal". Maybe someone with info on newer rigs can chime in.

On the Yukon 900s we have to pull the bumper cover to get at the condenser. It only adds a few hours labor to the job when doing it at a shop.

 

[OR VietVet]

Several of us have either picked crud in the system or low refrigerant. Knowing pressures each time there was a problem, would have helped but getting info from shops can be like pulling teeth sometimes.

 

[OR VietVet]

On the Yukon 900s we have to pull the bumper cover to get at the condenser. It only adds a few hours labor to the job when doing it at a shop.

Piss poor design for condenser access. So glad for my GMT800.

 

[L8T BURB]

Hi team -

Just heard back from the GMC dealership with the diagnosis: they discovered that the connector hoses are leaking. It's a $3,000 job that will fortunately be covered by warranty. I should have some more information later from them today or tomorrow.

Any update here? I am anxious to hear what the findings and resolution was.

 

[Lonny]

Any update here? I am anxious to hear what the findings and resolution was.

Well, the good news is that we're approximately five days back from the dealership and everything appears to be working, FINALLY! The hoses appear to have been the culprit. It's disappointing that the other repair shop wasn't able to sniff this out when they were replacing condensers and compressors. Hopefully this is the fix we need. In about two months we'll be into the hottest weather of the year here in Southern California so that will be the real test of the system.

This whole situation has been a good test of the extended warranty I got when I bought the Denali. Aside from the two $250 deductibles I had to pay out of pocket, they've covered everything throughout this nightmare and I just got reimbursed last night for my five rental cars.

It's also forced me to dig into the warranty to understand transferability, the expiration timeline, and my "fleet replacement" plan. The warranty coverage will be in place through December 2026, so my game plan is to keep the Denali through Q4 2025 while it still has a full year of coverage in place.

I started looking at the newly refreshed 2025s these past few days. Kind of amazing that the new 2025 Suburban and Tahoe are going to have Escalade-esque interiors and touchscreens! In my mind it kind of devalues the appeal of the pre-refresh 2024 Denalis. Much to consider in looking at the options.