#21 Fuse, Otherwise known as the Keep Alive fuse for the cluster
 

Okay 2006 F150 5.4 engine, followed some forums and tracked down my battery drain issue. Question is what's really wrong? So here's what happens; Normal conditions truck runs fine if I stop and shut off the engine leaving the #21 fuse in my battery will only last about 4 hours. It's a Di Hard Gold and I have checked the battery it's not the issue, as if I park and remove this fuse my battery will last two weeks at least. So can someone please tell me why in the world this 15amp fuse is killing my battery?

Thanks

Keep alive fuse? Is that going to be the fuse that allows the radio and cluster to not lose their set functions? If so then there may be a short in the instrument panel or radio.

It's fuse #21 I'm not exactly sure what it controls however it's obviously an always hot fuse. It's like whatever was shutting it down has broken now, and nothing is shutting it down.

Does your owners manual or the fuse box cover tell you what the fuse powers? There are several fuses that stay hot even with the key off. #21 may be one of them. Whatever it controls may have shorted out. Or somebody may have tapped into a line controlled by that fuse that is causing the draw. Are you the original owner?

I have the exact same problem. This truck has a brand new Motorcraft battery. I pulled the aftermarket radio out and am drawing .31 amps. When I pull the fuse, the draw goes down to .02 amps which is normal. My battery will last maybe a week to ten days with fuse in. No one posted a solution. How can I rule out if it is the key ignition switch? If okay, then what? Do not respond here as I started a new thread titled 2004 Ford F150 Parasitic Battery Drain. Thanks.

I have the same problem! 2008 F150 with 0.5A draw. I located the problem today as fuse 21 but I'm not sure where to go from here. What did you ultimately find out about your truck? Any solution? I don't think it's the ignition switch because mine is new. I think it has something to do with my rear overhead light. Maybe the 3rd brake lamp leaked and shorted it or something. I had some flickering of the light. If you found the answer please let me know. Thanks!

Fuse #21 is the Cluster Keep Alive fuse.

My issue ended up being the little rolling knob next to the steering wheel that controls the brightness of the cluster lights. I'm not sure if it is a loose connection or what but when I moved the knob up and turned on the cargo lamp it came on and then after I turned it off things were back to normal. When I press unlock on the key fob the cargo light comes on and when I open the door my interior dome light comes on. No more parasitic draw! I also sealed the third brake light using silicone because leaking is a known issue for the Ford F-150's. Hope you find your problem and get it fixed. Good luck!

Some on here have indicated a bad BCM
 

Body control module . They sent it off for rebuild to solve this problem . I have no first hand experience on this . Take that for what its worth . Good luck .

I have a 08 4.6L and I have been having a issue similar to that being the left cargo light being out as well as the factory radio display is very dim. Recently replaced fuel driver control module, truck didn't start up immediately but after sitting two days it started right up, spun aroubd the block a couple times parked and shut it off. Well......it started back up and for some reason it felt like it idled smoother but key felt "heavy" when I turned key. We decided to drive it only to make it a half block before it died while driving ig. Got it re-started and back home where it took a few times of re-starts and a high rev 4000 rpm start to get it back in place. Any insite greatly appreciated. My wife and I both love this truck but I refuse to spend $ equvialent to doubled mortgage payments. lol