Comfort access/keyfobs don't work after about 30 mins - battery registration problem?

[SGM E92]

Hi guys - I've got a situation where my comfort access isn't working, and I can't unlock/lock the doors with the fobs - I've replaced the batteries in both the keyfobs twice now (one set of energizer batteries, one set of duracell) and both times I thought the new keyfob battery fixed both problems but then within about 30 minutes of seeing it "work", CA and unlocking/locking ability quit working again.

2008 E92 DCT, 25k miles. I've owned the car since Jan 2015 and I know there's never been anything wrong with the car (I know the PO, have all the records etc.)

I just got my car back from the body shop, they replaced my front bumper cover after it got scuffed in a parking lot (no actual damage) but while I was having them do it I asked them to remove the license plate holder the PO's dealer put on without asking. They forgot to do that one thing right and I wasn't interested in them filling in the holes, so they had to order a new cover and redo it all so the car took about 2 weeks to get back instead of the 3-5 days it should have taken, and in the interim they said they had to recharge my battery to get the car to start up.

The shop only ever had one fob (my main one) but the secondary key does the same things. I don't know for sure that they didn't re-register the battery as 'recharged' after doing it, but they're also not an M3/BMW service shop so I wouldn't be surprised if they had forgotten to do that, but they do work almost exclusively on higher end cars and I was under the impression this was pretty standard with E90's of all stripes.

I'm also getting a phantom "left brake light is out" alert on the dashboard, but the light seems to be working fine.

I haven't cleared any codes in Carly because I don't know if doing that will make it harder for the shop to diagnose. I'm also not positive how long these codes have been there/present with no actual issues because I bought Carly while the car was in the shop and this is my first time using it.

: Brake light failure
Code 00A8C4
: Defective left brake light
Code 00A8B6

And under Instrument Cluster:
: CAN Signal fault transmission
Code 00A3AA
: supply voltage failure
Code 00931D

I also have, under the Audio/Navi/Communication tab:
Phone/Telefon (I assume this is not related to my issue, but since it says 'unlocks'...)
: Long and / or frequent Unlocks
Code 00D691

Doubtful, but not sure if that has anything to do with the actual locking system so I included it.

Does this sound like a battery registration issue?

Since this shop isn't a BMW servicer per se, I'd rather go in telling them what's likely wrong than letting them guess, so any advice would be very appreciated.

Just seems really weird that replacing the keyfob battery seems to 'fix' the problem but only for a little while.

[jdamore]

Please clear this up.. did you have the car battery replaced? You only register the battery when it has been replaced. If the car battery is week, the car will sense this and turn off modules (like comfort access) to save enough juice for one more start. Most likely, you are getting "Discharged battery" warnings on the nav screen (if you have one)?
Funny things can happen when your battery is near death. All sorts of electrical gremlins. With a 2008, if you haven't replaced the battery, its time to do it.

Also, check your tail lights. Are all the lights on? Check and clean all electrical connections. The seals wear out and water will get in and cause corrosion. Could just be another symptom of the battery..

[SGM E92]

The battery was not replaced, but it is 3 years old at this point. I was a little surprised the shop said they had to recharge the battery completely (as in, it was totally dead/wouldn't start at all) but the PO did keep it on a tender all the time because he'd told me the battery goes dead if you don't use it regularly enough just from the computer/systems/etc draining the juice, but because I drive it much more often than he did I have not had that issue. So I believed that the car sitting for 2 weeks might have done that to the point the shop needed to recharge the battery. (I trust them.)

I have seen the 'increased battery discharge' warning on a friend's car so I know what you're speaking of, but have not had that on my Nav screen before/it isn't showing up over the last week of this issue.

I thought perhaps the difference between a '3 year old battery' and 'fully recharged' was the issue, which is why I mentioned registration even on an existing battery, I am aware that 'weird electrical stuff' is generally battery related but I'm certainly far from an expert!

I'm puzzled because I have now 'solved' the comfort access problem twice by replacing the fob battery, I wrote off the first one replaced (one fob only) as a randomly bad Energizer straight out of a package I already had (not sure how old they were) but bought 2 brand new Duracells for the second attempt, which fixed it again - for the same 30ish minutes or so that the first 'solution' did.

One thing I just noticed after having just run another Carly diagnosis, when I got the car back last tuesday the battery on the report (at the very top of the Carly Fault Report) said '13.8V' and now, after driving it for a week the report reads '14.2V', not sure if this is just margin of error on the diagnosis or what (I'm very new to Carly, and haven't done anything much with it other than read these codes.)

E: Oh, and the tail lights all seem to be working fine as well.

[houston]

keep an eye on your EDC module.. known battery killer. If you start getting "increased voltage discharge" warnings unplug the EDC module to see if it goes away after a few days


Not totally related but low voltage battery can cause some strange errors.

[SGM E92]

I'll keep an eye on that, thanks.

After some further fiddling last night I was able to get the comfort access start to work if I'd put the key into the slot and then remove it, but only 'sort of' --
Still no lock/unlocking the doors with the fob, and after starting up with no key in the slot, I'd get the 'The remote key is missing or faulty' alert.

Also, sticking my finger into the slot for the key and poking around at the back of the alcove I was able to make the ignition switch between 'completely off' and 'ready to be turned on' status, but nothing was obviously wrong.

I started to wonder if they tried yanking the key out of the slot instead of pushing it to eject, but I don't know how that could affect the ability to unlock/lock the doors with the fob. They're also used to high end cars so PRESUMABLY don't just force things when they don't understand them... Hopefully anyway.

I guess I'll be calling them and reporting this issue since they have a lifetime guarantee on their work (and obviously this isn't how the car went into the shop) - just had hoped to have some ideas to give them but it sounds like the battery/registration theory I had is probably not valid/related to this problem.