Brake lights stay on (except third light) while truck was parked

BRIAN BRINK

MEMBER
2016 GMC SIERRA
6.6L
4WD
AUTOMATIC
46,000 MILES

I went out to my truck one night and the two taillight assembly brake lights were on, and they won't go off unless both battery grounds are unplugged. The third one comes on when the brake is applied. I noticed a deceased rat by my truck before this. Did he chew another hot wire that is touching those hot wires? Flasher module? Help. Please and thank you.

9 Replies

KEN L

MASTER CERTIFIED MECHANIC
47,643 POSTS

Rats can cause issues with wiring, that's for sure. Can you look under the truck to see if there is any obvious damage? Let's start by checking the fuses # BCM 4 BCM 6 BCM 7 on the right side of the dash fuse panel. This guide can help, and I have included the fuse locations as well. If the fuses and wiring harnesses are okay the BCM could be going out but to be sure let's do a CAN scan which is easy to do, this guide and video will show you how. They have CAN scanners for about $40.00 now on Amazon.

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

and

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Check out the diagrams (below). Let us know what happens and please upload pictures or videos of the problem.

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Monday, June 27th, 2022 AT 10:04 AM

BRIAN BRINK

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6 POSTS

Would a scan from AutoZone show something? I'll check the other stuff first(tonight). I have a friend that has a SnapOn scanner but didn't want to bother him yet. Thank you for your help and I'll update ASAP.

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Monday, June 27th, 2022 AT 12:22 PM

BRIAN BRINK

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6 POSTS

So, I checked the fuses with the key on and found the LH side #33(spare) has no power on either side of fuse and on the RH side #10(cargo) has no power either side of the fuse. Otherwise, all other basic fuses on these panels have power on both sides.

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Monday, June 27th, 2022 AT 1:07 PM

KEN L

MASTER CERTIFIED MECHANIC
47,643 POSTS

Yep, that is normal. The power is supplied by the BCM through the fuse then onto the lights. I would do a CAN scan to confirm the failure. You can use the scanner for other car repairs as well.

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Tuesday, June 28th, 2022 AT 11:08 AM

BRIAN BRINK

MEMBER
6 POSTS

Looks like the Block. Fuse and Relay Box. Fuse And Relay Center is shorting out. I brought it to my buddy's shop and we scanned it and checked all around and it appears the fuse/relay box is the problem. None in stock locally as they seem to be having problems with them. Hopefully I will have it mid-week and I'll update if that is the fix.