HVAC issues on a 2021 3500hd lt

Behind Bars · December 9, 2021

I wonder if anyone else is having these issues. Since new this truck has some gremlin in the hvac. The fan will run full blast for an entire drive, sometimes with outside temps at/near parity with int thermostat setting. After a few minutes you sort of get used to the jet-blast sound until the phone picks up or I shut off the truck, then the silence is deafening. When I turn off the hvac the truck is so quiet I can't believe it! Also, at one point during the heat of the summer the a/c quit working entirely--the dealer said it was over-charged and I never had a problem again after they serviced it.

Then when towing this fall my truck entered the next faze--window fogging. I was towing a heavy trailer on a 50 deg dry day when suddenly the windows fogged up. Completely. It probably took 20-30 seconds from clear to entirely fogged over. The truck wasn't hot and there was no smell of antifreeze. After getting it to clear (defrost on hi, windows down), it was touch-and-go from there. If I turned off defrost it fogged over, at least while towing. Since then I haven't towed but have had several more fog-over incidents and have had to drive with the defrost on hi for an hour or more while the edges and side windows remained foggy the entire time.

I took it to the dealer who diagnosed it with a bad heater core. It didn't make sense entirely to me but there is probably much I don't understand on these newer trucks. They said they had two more (gas engine) coming in with bad heater cores as well. They had it for 4 days to change the heater core. On pick-up I asked the service adviser if the old heater core was leaking. She acknowledged it was in an off-hand way, so I re-iterated the question and was assured that the old core was leaking. Next day after hunting I drove an hour back with the windows fogged up.

Now they've had it for over a week and no news yet. Anyone else experiencing hvac issues like this?

There is no smell of antifreeze. The way it initially fogged during a dry day makes me think moisture is being introduced, like condensate is backing up into the system.

Here's another twist: the engine cooling fan has started coming on at weird times. It kicked on while coming down a pass in 35 deg temps, empty, going 40-50 mph. It came on the other morning before the truck was even up to temp. I can't figure out how they could be related but it seems peculiar that both of these issues are occurring. There was also a buzzing/vibration at idle ONE time and I discussed this with the service dept--they said there was a service bulletin for a fuel line rubbing on the firewall and that they would fix that.

Another JR · December 9, 2021

I have had my gas 3500hd slt for 8 months and 3500 miles, with no such issues.

On your fogging issue, have you verified that when it fogs up and you turn on the defroster that the a/c compressor is running?

Your cooling fan coming on after they replaced the heater core makes me wonder if they failed to fill and bleed the air out of the cooling system properly.

A bad temperature sensor telling the computer the engine is excessively hot might cause both the fan to come on and the a/c compressor to be locked out. That’s just a guess though - I have no idea if such a sensor or logic actually exists in the system.

Behind Bars · December 9, 2021

I did not confirm that the compressor was running. I was not clear but the fan clutch was engaging before the heater core was replaced--I noticed it a couple of times. I do not believe the fan clutch has any electronics--at least the service advisor said as much. I know the fan clutches I've worked on simply had a spring in them that caused them to engage when hot. I do not know if they fan clutch has any relation at all to the hvac issues but it seems suspicious.

It is a strange bunch of issues and I feel like the right mechanic, someone familiar with the hvac system and with problem-solving skills, would know right away what might be causing this. At this point I was hoping that this issue might have occurred elsewhere and been resolved.

Thanks for your help. I'm growing concerned that it could be a bad computer component/software that is causing the trouble.

Another JR · December 10, 2021

Is the water temp normal when this happens?

I’m pretty sure the fan clutch is a simple viscous clutch based on looking at the parts catalog, so no connection to any external sensor or computer.

Edited December 10, 2021 by Another JR

Behind Bars · December 10, 2021

Yes, normal or even low. I made a video I provided to the dealer when I could hear it engaging while the truck was still coming up to temp, maybe 180 deg. So I don't think the engine is in danger of over-heating, I think it just has a bad fan clutch. It is probably unrelated to the hvac issues.

TheElementalCashew · December 13, 2021

I just had this happen a couple nights ago in my '21 AT4. I was driving along and suddenly the windows all fogged completely. After they cleared, it happened a couple more times very quickly, and then very quickly cleared on their own.

I have also noticed, now that the weather is colder, that the truck doesn't really seem to heat up. I usually run 74* on auto, and I can drive for 45 minutes and still be comfortable in a jacket. Not sure if these things are related, and not really wanting to leave it with the dealership for diagnosis.

shakenfake · December 13, 2021

74 is not even warm so yeah that makes sense lol

TheElementalCashew · December 13, 2021

Maybe....74* in my 1500 AT4 would get me sweaty, but very different trucks I suppose.

BlaineBug · December 13, 2021

Check in your climate control settings. The previous generation had a setting for "AUTO FAN" where you could select medium, low, or high. I set mine to LOW. Check and see if your various menus have a similar setting.

EDIT - I am now confused if you're talking about the engine fan, HVAC blower fan, or both. Regardless, it is possible your engine fan would come on at a "weird" time because when you do have the HVAC settings in auto mode, generally the A/C is turned on to help dehumidify the interior and windows. Therefore the front condenser may require more airflow to increase A/C efficiency, and since the same fans that flow air through the condenser also flow air through the radiator, there isn't much option there.

Edited December 13, 2021 by BlaineBug

newdude · December 13, 2021

Haven't seen bad heater cores yet at work. We have seen a bunch of clogged HVAC evaporator drains. Mostly on 1500's but they use the same exact drains and heater boxes. Heater box gets water inside it as a result and could explain the moisture you are seeing.

Leaking heater core you typically will get coolant smell from the vents.

Behind Bars · December 29, 2021

The dealer had my truck for over 3 weeks. They say they located and fixed the issue. Apparently GM required them to tape over the ducts/vents throughout the truck and pressure test the cab for leaks, something they seemed very reluctant to perform. On doing the test they found that some cab vents located under the back seat were mis-aligned and so blocked, I guess from the factory. I understand they are to let air out of the cab, I suppose to have new air in you must let air out.

According to my service shop, GM is aware of some issue that is not exclusive to my truck.

The engine cooling fan still ran for a short time during really cold temps just after entering the freeway but came back off again and then cycled briefly again in an hour highway drive while empty. But my windows have yet to fog up again.

On 12/13/2021 at 7:01 AM, newdude said:

Haven't seen bad heater cores yet at work. We have seen a bunch of clogged HVAC evaporator drains. Mostly on 1500's but they use the same exact drains and heater boxes. Heater box gets water inside it as a result and could explain the moisture you are seeing.

Leaking heater core you typically will get coolant smell from the vents.

Newdude, can you describe where I can find the hvac evaporator drain or tricks to keep it clean? Sorry, I've not looked nor do I think I've had a vehicle before with a drain location. The sequence you describe makes perfect sense as to what I've seen. As well, could ice form there and block it temporarily only to melt later, causing confusion?

Also, there never was coolant smell in the cab, which I told them, but the service rep still claimed that the heater core was replaced and that the old one was leaking.

On 12/13/2021 at 6:34 AM, TheElementalCashew said:

I just had this happen a couple nights ago in my '21 AT4. I was driving along and suddenly the windows all fogged completely. After they cleared, it happened a couple more times very quickly, and then very quickly cleared on their own.

I have also noticed, now that the weather is colder, that the truck doesn't really seem to heat up. I usually run 74* on auto, and I can drive for 45 minutes and still be comfortable in a jacket. Not sure if these things are related, and not really wanting to leave it with the dealership for diagnosis.

I've constantly been changing the temp in this truck. In the 4 previous duramax trucks I've owned it's been set-and-forget but not this truck. The way you describe it fogging, clearing, and fogging again sounds just like what I have encountered. I don't know anything about the vents under the back seat but moisture build-up in the heater box, as described above, certainly would account for this issue.

Behind Bars · January 3, 2022

The windows still fog. I drove the truck about an hour without trouble, stopped to fill up the sleds with the truck at idle for about 10 min, then hit the road and the windows fogged immediately. The interior was completely dry and all gear/clothes were dry. It fogged up and stayed fogged at the side windows and perimeter of the windshield despite running the defrost on max for 30 min straight, with max fan speed. Finally I put the windows down partially and that helped some. However, at 16 deg outside temp that get tiresome as does driving with the fan on hi constantly.

Driving back I turned off the AC while running the defrost and it never fogged up again. I drove up and back again with the same results.

My guess is that moisture is building up in the system from condensate perhaps from the heat exchanger on the a/c and being introduced into the hvac system. Maybe it drains but freezes in cold temp, though this problem began during 50 deg temps.

BlaineBug · January 3, 2022

20 minutes ago, Behind Bars said:

The windows still fog. I drove the truck about an hour without trouble, stopped to fill up the sleds with the truck at idle for about 10 min, then hit the road and the windows fogged immediately. The interior was completely dry and all gear/clothes were dry. It fogged up and stayed fogged at the side windows and perimeter of the windshield despite running the defrost on max for 30 min straight, with max fan speed. Finally I put the windows down partially and that helped some. However, at 16 deg outside temp that get tiresome as does driving with the fan on hi constantly.

Driving back I turned off the AC while running the defrost and it never fogged up again. I drove up and back again with the same results.

My guess is that moisture is building up in the system from condensate perhaps from the heat exchanger on the a/c and being introduced into the hvac system. Maybe it drains but freezes in cold temp, though this problem began during 50 deg temps.

Next time try turning A/C off manually. I normally press "auto" then "sync" then press the A/C button, so that the A/C light as well as the "auto" light turn off.

If you want to test that it is in fact the A/C evaporator that is contributing to your moisture issue.

Behind Bars · January 10, 2022

On 1/3/2022 at 10:15 AM, BlaineBug said:

Next time try turning A/C off manually. I normally press "auto" then "sync" then press the A/C button, so that the A/C light as well as the "auto" light turn off.

If you want to test that it is in fact the A/C evaporator that is contributing to your moisture issue.

I've been running it with the A/C off since and have not had any issue of fogging since so I assume the condensate was the problem. When I get a chance I'll climb around under the hood and find the condensate drain myself and see what's going on. Maybe I'll go ahead and replace the fan clutch myself as well as I'm sick of listening to the engine fan run everywhere I go. I think if I have to wait for the warranty to perform I'll be waiting a long time.

Adam9066 · January 12, 2022

I notice my engine fan kicks on at weird times as well. But I link that to the fact that my truck has the front grill shutters and they are probably closed causing the fan to have to run to cool the radiator.

I have noticed on cold days they tend to stay closed during warm days they are always open.

Edited January 12, 2022 by Adam9066