2015 SCREW 5.0 to get that out of the way. I've been having weird random electrical issues for a while now. Don't remember all that was going on but it prompted me to get my battery tested, which ended up being bad. Replaced battery about a month or so ago (maybe not even that long) and things quieted down for a bit. Late last week and this weekend the truck started honking once, then the flasher would turn on (while truck was parked/key out). Last night I unhooked battery so it wouldn't do it again and wake neighbors overnight. Woke up this morning, put negative cable back on battery and the horn started blaring nonstop. Finally stopped after a minute or 2 of idling but then I got emergency brake notice (brake was physically not on) and tire pressure warning, none of the monitors are communicating with the truck. Ebrake message finally cleared, tire message is still on. I parked truck after a 5 mile drive for dentist, horn started going off solid again after about 30 minutes. Again got it to stop. I installed a subwoofer using all Ford cabling but I'm wondering if that is staying on and wearing battery down so I unplgged it to eliminate that issue.
My plan right now is:
Leave sub unhooked
Get new battery tested
observe
If problem seems to clear I'm going to hook sub back up in plain view and check to see if power lights ever go out
If problem persists I guess I'm looking at the BCM?????

I hooked up the laptop and checked DTCs yesterday, nothing but a "general electrical fault" calling out the inverter (my AC plugs do not work so I assume that is it). I don't remember code but I'll check again tonight after this horn issue

 

Check the BCM for water intrusion. There has been a number of postings where this has been the issue. The cause can be: poor quality windshield replacement; leaking cowl seam right from the factory; or disconnected Vista roof drain line. Also, there is a ground just to the aft of the BCM which is the major ground for the BCM. If that is loose it can cause all sorts of weird things to happen.

 

I've seen the water intrusion posts. I've never noticed any water over there but I will take a closer look at that and the ground this evening. Thanks

 

Is the BCM in the passenger side foot well, near the fuse panel?

 

Yes. The fuse panel is actually an integral part of the BCM.

 

ADD to 52merc's #2 post, check ground #303 - pic below

 

And to add some more - check the CHMSL area for leakage as well.

 

How long have you owned it and when did all this start? Did it start with a new battery replacement.

I'd also check out the fuses. Might have one or 2 that are bad. That and a weak battery might be in play.

 

You may have low battery voltage even with a new battery if you didn't reset the BMS. If you don't reset the BMS it doesn't know it's a new battery and will charge it like the old battery which may not be enough.

 

Have owned the truck just under 1 1/2 years. I was getting some weird electrical issues and the old battery tested bad (bad cell). After changing the battery things seemed to work fine for a while and then these issues with the flashers, horn etc started up. Stopped at Advance yesterday and battery tested out fine. I saw some hits that mentioned USB chargers occasionally draining battery so this morning I have everything unplugged. Checked the BCM area for water but didn't notice anything, looked good to me. Verified both grounds mentioned are tight.
When I replaced the battery I did the key method of resetting things. After researching yesterday I did realize I didn't change the battery type to AGM (went with AGM on new battery) so I will be doing that tonight or tomorrow