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I tried searching this topic but didn't find too much that was useful.

Both of my key fobs no longer work (which stopped working right around the same time as my PDC). I looked @ the fuses and none appear to be faulty. Is there a way to to replace the battery in the key fob? I took a screw driver to it, swiss army knife, letter opener, binder clip to it and nothing seems to pry it open. Can the key fob even be opened without cutting it open and soddering a new battery in? Is there some place else I should be checking to address the issure of my key fob no longer working? Mind you, when I open the car the old fashioned way by actually using the key, it works fine...
 

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I'd re-initialize the keys first (search the directions here).

If that doesn't do anything, then lock the doors with the key. Check the gas door and see if it opens. If it does, then you have a defective gas door actuator. For some reason on the E39 and E53, if the gas door actuator is not working, the remotes are disable.

If the gas door is locked, then I'd check the fuse.

If the fuse is good, you just might need to get a new key.
 

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It seems unlikely both keys' batteries (or capacitors) have failed at the same time. More likely they have lost sync with the car. I just learned about this on another thread the other day. What you do is get in the car, insert the key and turn to position 1 and remove the key. Then hold down the unlock button on the fob while simultaneously pressing the fob's lock button 3x, then release the unlock button. Then do the other key, as doing one key will disable the other one, so you need to do them both in succession (do the second one WITHOUT sticking the key into the lock again - I just tested this on my M3, which had one of the two keys lose sync. If you do one, then stick either key back into the ignition, it will just re-initialize the second key and disable the first one. To get them both to work, do one, then just do the hold button thing with the other key immediately afterwards without putting anything into the lock.). If this doesn't work, and since you've already tried and found the fuses and gas door working ok (try that), then you may well have bad batteries. I think there is no user replacement but you need to buy a whole new unit.

Why this is necessary seems to have to do with the rolling codes as between key and car.
 

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Do first what Mikeymike suggested.
Then do the re-sync like Need4Speed suggested.

If that does not work,look into the keys.It is possible to open them(you have to cut them careful open with a knife,very interesting experience,btw...)

I repaired a few diamond shape keys like our beasts have.Most of them it was not the rechargeable battery that was done(is available online cheap).

In most of the cases the latches from the battery to the board broke.Only good fix is to replace the battery as resoldering only works temporary and it breaks again(after resoldering one of my keys the 3rd time,I just bought a new battery)
 
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