2020 Silverado Trail Boss - switches to sport modes, trailer towing mode, 4wd high and 4wd low on its own. Dealership is stumped

wacnstac · January 30

I have a 2020 Silverado Trail Boss which switches to trailer towing mode, all the different driving modes, 4wd high and low all by itself while driving down the road. I can typically switch it out of these modes and back to normal by the switch on the dash. Last straw was when it started trying to switch itself to 4wd low while at highway speed. It didn't happen but was scary enough to take it in.

Dealership seems stumped. They are talking about "rear actuator or rear carrier" and neither one of those make any sense to me. Seems like they would be talking about a transfer case module or transmission module.

I'm concerned they are on the wrong track since all of these things are controlled by the switch on the dash, logic would say it was the switch or wiring to the switch. They claim they have swapped in a different dash switch and the problem still happens.

Searching the internet doesn't yield a lot of info on a problem like this.

silveradosid · January 31

i would be checking the wiring harness, there is a tsb on where the harness is known to rub through and cause problems. also check the ground strap behind the front right wheel thats behind the wheel liner

wacnstac · January 31

Any idea what the TSB number is? Dealer doesn't seem to be aware of any of this.

newdude · February 1

Bad switch in the dash? The drive mode selector and 4x4 controls are all a part of one single switch assembly. Sounds like there could be an internal fault with that switch seeing its not only trying to shift 4x4 modes, but its changing to tow/haul mode (and sport and off road if you have/its doing that).

Did they actually "swap the switch" for another to test? Any codes present? It doesn't seem like it would be a transfer case control module or transfer case encoder motor issue as its changing drive modes, and it doesn't seem like a TCM issue as its trying to control 4x4 modes and the TCM has no control over shifting the transfer case.

4LO won't switch when moving anyway, truck won't allow it if you are moving in Drive or moving at any speed aside from 0-1mph.

wacnstac · February 1

12 hours ago, newdude said:

Bad switch in the dash? The drive mode selector and 4x4 controls are all a part of one single switch assembly. Sounds like there could be an internal fault with that switch seeing its not only trying to shift 4x4 modes, but its changing to tow/haul mode (and sport and off road if you have/its doing that).

Did they actually "swap the switch" for another to test? Any codes present? It doesn't seem like it would be a transfer case control module or transfer case encoder motor issue as its changing drive modes, and it doesn't seem like a TCM issue as its trying to control 4x4 modes and the TCM has no control over shifting the transfer case.

4LO won't switch when moving anyway, truck won't allow it if you are moving in Drive or moving at any speed aside from 0-1mph.

Thanks for the great response. Dealership claims they tried another drive mode selector switch on the dash and the same problem occurred. I know that a 4LO switch is prevented when moving but having that come up at highway speed is very disturbing. They said there are some codes present but of course they didn't tell me what they are.

When the vehicle does change modes on its own, it always obeys what I do afterward with the dash mode selector switch at least for a while.

Edited February 1 by wacnstac

newdude · February 1

8 hours ago, wacnstac said:

Thanks for the great response. Dealership claims they tried another drive mode selector switch on the dash and the same problem occurred. I know that a 4LO switch is prevented when moving but having that come up at highway speed is very disturbing. They said there are some codes present but of course they didn't tell me what they are.

When the vehicle does change modes on its own, it always obeys what I do afterward with the dash mode selector switch at least for a while.

Ask them for the codes. Or if they ran a SAVI report, ask them for a copy. Dealers are required to run SAVI reports if its under warranty but most of the time they run them just because.

If they won't give you the codes (don't know why they wouldn't), do you have another dealer nearby that can print you out a SAVI report for your truck? Or a trusted repair shop that can scan the truck? Or a scan tool yourself that can read codes for the entire vehicle?

Edited February 1 by newdude

wacnstac · February 3

I had changed the battery to a non-OEM but AGM battery two winters ago. Now I'm paranoid that the change may have caused some of these problems.

Dealership was focused on a bad Transfer Case Control Module and I am not convinced. The TCCM but I don't think the TCCM is involved in trailer tow mode or the different sport modes which the vehicle likes to switch between on its own. I told them if they don't find anything wrong with the switch on the dash, or the harness to it and verify the BCM and TCCM software versions, and a CAN bus ground inspection, I just want the vehicle back.

Edited February 3 by wacnstac

wacnstac · March 5

Dealership was of zero use in debugging this problem. No codes = no fix. The truck has now tried to put itself into terrrain mode randomly. This mode is not even available on this truck as the truck has 4WD Low! and there is no separate switch for terrain mode or an option for this in the menu.

I'm thinking this definitely has to be an intermittent mode selector switch. I think the way to narrow this down would be to remove the switch from the harness. I am under the impression, that I could drive the truck this way knowing that I would not be able to change the modes obviously.

Edited March 5 by wacnstac

newdude · March 5

1 hour ago, wacnstac said:

Dealership was of zero use in debugging this problem. No codes = no fix. The truck has now tried to put itself into terrrain mode randomly. This mode is not even available on this truck as the truck has 4WD Low! and there is no separate switch for terrain mode or an option for this in the menu.

I'm thinking this definitely has to be an intermittent mode selector switch. I think the way to narrow this down would be to remove the switch from the harness. I am under the impression, that I could drive the truck this way knowing that I would not be able to change the modes obviously.

Pull it and try driving. See what happens. Worst case it turns the check engine light on maybe? I suppose it could limp mode it but honestly, I've never driven one with the switch unplugged lol.

wacnstac · March 5

I'm hoping it wouldn't even know the switch is unplugged.

Jay P · March 5

Check this out. It caused some weird electrical issues with the 19 Sierra I had at the time. May not be your issue but pretty easy to check.

MC-10178357-9999.pdf

Edited March 5 by Jay P

dennis faulkner · July 30

My 2021 Trail Boss has the same issue of switching drive modes. I just replaced the switch and the issue happened again after 2 days.

dennis faulkner · July 30

Has anyone found a solution?

dennis faulkner · July 30

On 1/31/2026 at 7:03 AM, silveradosid said:

i would be checking the wiring harness, there is a tsb on where the harness is known to rub through and cause problems. also check the ground strap behind the front right wheel thats behind the wheel liner

Do you know where and which harness is rubbing?