Electrical gremlins multiplying 2009 Tahoe LTZ

[PatDTN]

For a while now I've been having problems with the HVAC in my Tahoe. If the Tahoe sat for more than a week the selections would "default" to 74 degrees, Rear on, Passenger on, and Auto off. Normally it's set for 72 degrees and Auto on but the other two things off. As time has gone by the length of time sitting for the problem to appear has gotten shorter to now about a day and a half.

Yesterday I discovered that my high beam flash function doesn't work.

Is there a battery somewhere besides the main battery that might maintain settings on HVAC?

Is there any way the high beams could be related to the HVAC?

Is all this in the BCM and I'll start having bigger problems soon?

 

[PatDTN]

so, a new development. I stopped in a friend's driveway to chat with him and turned the key to accessory to leave the radio playing. Listened and talked for a couple minutes then turned the key all the way off. Sat and talked for about 15 minutes. Never opened the door. When I turned the key to start the Tahoe back up it clicked and I watched the dash lights come on and fade out.

The battery I bought in November of 2018 from Walmart has given up the ghost it seems. Oddly it was always happy to start the truck before this. I put a battery charger on it and let it build a little voltage but the charger wasn't indicating it was doing much so I kicked it over to the 50 amp start position and started right up. Gauge was reading 14 volts as i drove home.

I'll do some searching around this site but does anybody know if I can use an AGM battery and just drop it in?

 

[Doubeleive]

load test the battery and check the battery cables some are known to corrode from the inside yet look ok on the outside, you can check for resistance and for voltage drop.
loss of voltage/ground is what makes these trucks go goofy

 

[PatDTN]

Thanks. I'll have it load tested. In looking around I see I should upgrade the battery anyway since I dropped in the 6.2.

 

[PatDTN]

Okay I skipped load testing and bought the AGM battery at Advance Auto. They installed it and I noticed that it started even quicker than I've seen. On the drive home just for grins I tried the high beam flash and that worked. I'm feeling lucky on the HVAC thing next.