I used the ebay kit for my spare key that had a dead battery when I got the car. I'm not good at soldering but got it done quick enough. It comes with spare micro switches if I remember correctly, though for me changing the battery was the answer. I had the key cut at a local key shop.

Getting back to this topic and finally about to order the Chinese made remote key fob from Ebay as advised to see if the problem is in the key or the car itself.

Not sure if I'm mistaken, but if I would order a remote key fob like this: https://www.ebay.com/itm/274720812901 and proceed with the typical programming operation by putting the key blade into the ignition, isn't there some kind of a theft protection which detects if something unsuitable if inserted into the ignition switch and then make it impossible to start the car afterwards? At least I recall there was something similar to this in the door lock of the E53. Or should the blank key blade be safe to use in the ignition?

You can put the real key in the ignition to stay the pairing process. You then treasure it out but do to button presses with the new fob.

There are two separate radio signal paths in the key one that uses the battery and one that works like RFID and is powered by the incoming radio signal.

Ah I see thanks for the info! I’ll order the key fob today, let’s see how it goes.

A lot of people will get a cheap $10 eBay key and just add to their keychain as a fob for remote access. I bought a real key from online dealer for a significant discount but still like $160

Reporting back since I have now received my Chinese remote key fob and would like to have some insights if possible.

So afterall, I was unable to pair the new remote key fob with the car, which makes me believe the issue is in the car itself and the keys can be ruled out. Quick recap of the situation:

-I have two original keys for the car which both I have been able to pair with the car.
-Currently only one is paired.
-The remote door locking/unlocking and remote trunk opening only works intermittently, lets say 5-10 % of the times when pressing the buttons. Although sometimes it feels it doesn't matter how many times you press the buttons it just doesn't work and you are forced to unlock or lock the door lock manually.
-When I got the car last August, the other remote fob was working perfectly, the other didn't, perhaps it wasn't paired with the car. Then after few months it began to show issues, first only every now and then but eventually got to this point. I don't think it has gotten any worse in the last few months.
-Both keys have been checked by a company which specializes in car keys and according to them, should be fine now. The batteries were fine in both of the keys, but the other one wasn't sending a signal at first but this was fixed after adding some solder.
-Central locking button inside the car is working perfectly

I guess it is still somewhat possible that the issue is in the keys and maybe the Chinese key fob is just faulty to begin with but I really think I should now start investigating the car itself. Is there a receiver somewhere which could be checked or maybe the wiring?

Remote key working intermittently
 

I went though the same. I reflowed the solder on both my keys. They worked again for a while. Then they stopped.

The weirdest part was: key world work if I slammed the trunk but that even stopped working.

I eventually "bit the bullet" and ordered a real key from online dealer.

Fixed the problem 100%.

It was exclusively the key the entire time! I suspect the same in your case. The original keys have enough bad solder joints they aren't able to sync with the car anymore and the cheap copy just isn't up to snuff.

I was ecstatic when I finally just did it. Worth every penny to have a real key. There is an online actual legit BMW dealer that will save $60 or so. I bought a valet key with the savings.

Search xo for the thread on the subject. It has links to the exact part webpage.

I just need to do the same thing, buy new keys from BMW.

Thanks for this entire thread as I’ve been following it closely. Very occasionally my key will fail to work at all, however after I unlock the door manually with it, the thing starts working again. Sounds like the eBay/Amazon stuff isn’t worth the time for this.

This was my exact symptom that led to me doing a reflow on my two keys.

If I locked or unlocked with the key blade the fob would immediately work for a few presses lock/unlock.

I could also get it to work after a battery jump start making me think it was possibly worn battery but after opening up and measuring battery = good I reflowed most of the solder joints and both keys syncd after that and worked for over a year

Thanks for the opinions, this whole key issue indeed is rather mysterious. Maybe now I just have to buy a new original key from BMW at some point. After all, I have never heard anyone having a faulty receiver. Chinese Ebay key fob was worth a try though since it was so cheap.