BMW Key Fob Doesn't Work
Last updated December 11, 2015
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First, let me just say that this only applies to cars that use the following two keys. These include the E46 3-series, E39 5-series, E38 7-series, E53, X5, E83 X3 and E85 Z4.
There are many reasons why the key fob doesn't work with the car and I'll try and touch on most of them. Now this page is for fobs keys that don't do anything, that means: no locking, no unlocking and no truck release. The key fob may or may not start the car, that's part of the EWS system and I am not going to discuss that here so if that's your only problem then you can stop reading now.
The key fob system is called the FZV system (don't ask me what it stands for). The system consists of the key fob, the antenna amplifier and the general module. When the button on the key fob is press it basically sends out three things, a unique identification code, a pseudo random rolling code and a button code. The antenna amplifier receives the signal (typically transmitted at 315 MHz) demodulates it and sends a baseband signal to the general module. The general module looks at the unique identification code to see if it is on the approved list, if it's on the list it takes the last received position in the rolling code sequence and compares it to the current received position. If the current position is within 1000 of the last received then it is considered good and the current position is saved as the last received. The button code is then looked at, decoded and acted upon.
The most common failure is probably a loss of position in the rolling code sequence by the key fob. This can happen because the battery in the fob was replaced or went dead from non-use or what I find is it's broken trace, component, connection in the key fob. The position can be resync'd by performing the key sync procedure <- follow the link for instructions on how to perform this procedure. Next would be a failure of the antenna amplifier <- follow the link for more information. This doesn't fail nearly as often as the key itself. Last step is the general module. I have never seen a failure of the general module cause the key fob to now work without many many other failures also appearing at the same time. This is only because of complete failure of the general module processor. If your problem is only unlock then you're on the wrong web page. That's a common failure on cars with the general module 5 and is not typically a key fob issue. General Module 5 does not unlock.
Ok, back to the key fob. In many cases just performing the key sync procedure will fix the problem, but, why did it loose sync in the first place. If you are outside of range of the car and press the key fob >1000 times it will get out of sync. While possible it typically is not what happens. Usually it's what I mentioned before, battery, broken trace, component or connection in the key fob (for the diamond key it's typically not a bad battery). Just so you know, loss of power to the general module will not cause it to loose the rolling code positions. I can test and repair the key fobs. If you have two key fobs and they both stopped working at the same time and key resync doesn't work then it could be a bad antenna amplifier. I can test and repair the antenna amplifiers as well.
Key sync doesn't work. The most common problem for key sync to not work is one of the doors is open during the procedure. All doors much be closed. To do a quick check, turn ignition to On and check the dash board indicator for an open door. Next, is a key falsely detected in the ignition? To do a quick check, set the headlight switch to Off, close the doors, insert a key and turn the ignition to On, then turn back to Off but leave the key inserted. Now open the door. The car should beep at you, now remove the key from the ignition. The car should stop beeping. If it keeps beeping then something is wrong with the car and it's not the GM5 module. Next I've seen keys not sync because either the Lock or Unlock buttons on the key fob were bad. This is a repairable problem, key fob repair. There must be one or two other problems because there was one car that came in that I could not sync the key to the general module in the customers car. Everything above checked out, but, it would not sync. I know the key was good because I tested it. I took out the customers general module and installed it in another car and synced his key to his module in the other car just fine and when I moved the module back to his car it worked fine to lock, unlock and trunk release so I know the key is good, antenna amp is good and general module is good, but, just would not sync. That car is still a mystery to me.
I recently figured out a weird problem with a E39 5-series where I could not sync the key. Key was tested and found to be fine. All functions of the general module appeared to be fine except it would not accept key sync. When the module was on my test station I could sync the key to the module fine and when the module was moved back into the car the keyless entry system worked, but, it would not accept key sync. Fuse 38 (5 Amps) was blown. Replaced the fuse with a good one and key sync worked again. The customer also reported that the passenger side rear view mirror started tilting down when shifting into reverse.
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