Just took delivery of my 2021 Sierra Denali Saturday (3/20/2021). Problem I m having is when connected to my Battery Tender the horn goes off every few hours. To stop the honking, I have to push the lock button on the key fob. I’ve got the positive clamp connected directly to the positive battery terminal and negative clamp connected to the body ground which is on the frame next to the battery. The vehicle is locked in the garage with the hood up. The honking usually happens 4-6 hours after connecting the Battery Tender, then happens every 3-4 hours afterwards.
Immediately after the horn begins honking, I receive a text message from Onstar stating: “OnStar: 2021 Sierra theft alarm activated. Call OnStar 888-466-7827 or 911 if an emergency. Do not approach vehicle unless safe. Txt STOP to cancel.”
I went out this morning at 5:30 and the battery tender shows a fully charged battery (solid green light). I’m thinking after becoming fully charged, the Battery Tender goes into sleep mode. Then after a few hours, it goes back into trickle charge mode, and this is when the horn starts honking. And the theft alarm is activated.
Is there any way to prevent this from happening? Maybe keep the hood closed?