C4 Replacement Fob Not Working

[Paul-R]

You've said it better than all my waffling. I'd also like to try it with a S/H fob as it's my understanding that the fob doesn't get written to but I might be wrong on that.

[Cutac]

What's an S/H fob?
Because I've tried that on my B7 with both Gary(regular flippable fob, standard equipment) and a single key without electronics and Jimmy couldn't pair.

[GiveMeABreak]

You put a coded fob(Gary, stay with me:mrgreen:) in the ignition, twist it, car starts.

Perfect - that's normal.

You put a coded fob(Gary) in the ignition, hold the lock button and the car's remote reciever is paired with that particular fob(Gary).

Not quite - requires the ignition to be turned on, then the lock button pressed in for a few seconds - that's called re-synchronisation and has no effect on a non-coded fob.

Now what I wanted to do:

I put a coded fob(Gary) in the ignition, hold the lock button on an uncoded (brand new, separate, let's call it Jimmy) fob and now the coded fob(Gary) can still start the car, but the uncoded fob(Jimmy) is used to operate remote locking.

This is where nothing will happen. The 'Jimmy' fob cannot operate the remote locking because it is not coded. Neither can it be synchronised before being coded.

I get it that's not possible, but just to clear the air on my understanding of terms code and pair

I get that, and understand you, and my answer is the same

For the Remote Locking / Unlocking to work, the fob must be programmed (coded) first of all. Re-Synchronising is something different.

In the diagram below, you can see the Electronic receiver unit (4), and the High Frequency Antenna (3). When one of the fob buttons is pressed, it sends out the high frequency encrypted code, which is picked up by the Aerial (3) and transmitted to the electronic receiver which then talks to the BSI. If the encrypted code is correct, the BSI authorises the doors to be unlocked / locked.

Re-synchronising only resets the offsets when the maximum number are reached (circa 250+) caused by operating the remote at long distances / interference etc. This procedure cannot programme a fob.

[Paul-R]

For the Remote Locking / Unlocking to work, the fob must be programmed (coded) first of all. Re-Synchronising is something different.

That sounds like you're saying both the ID46 chip AND the remote transponder are being written to.

But, even so why cant that be done to a separate fob?

[Paul-R]

Re-synchronising only resets the offsets when the maximum number are reached (circa 250+) caused by operating the remote at long distances / interference etc. This procedure cannot programme a fob.

When you say fob, what exactly do you mean? The ID46 chip (which I accept is now part of the PCB), the remote lock/unlock circuitry or everything combined.

@Cutac S/H means second hand.

[Cutac]

This is where nothing will happen. The 'Jimmy' fob cannot operate the remote locking because it is not coded. Neither can it be synchronised before being coded.

That last sentence perfectly answers my question!

Paul, aren't second hand fobs in a way bricked if they've previously been coded to another car? Once coded, they cannot be re-coded, isn't that right?

[Paul-R]

Well I always thought it was the ID46 chip that was once only programmable. I didn't think that the remote lock/unlock was anything to do with the chip, indeed on earlier cars (such as the Mk1 C5 and Phase II Xsara that we used to have) the chip was a separate item. One of the keys for the Xsara just had an ID46 chip inside as it did not have the remote locking system.

I suspect that I'm not going to "get" it as I can't test all the possibilities I want to. I'm one of nature's sceptics who has to understand exactly how something works.

[GiveMeABreak]

This is where nothing will happen. The 'Jimmy' fob cannot operate the remote locking because it is not coded. Neither can it be synchronised before being coded.

That last sentence perfectly answers my question!

Paul, aren't second hand fobs in a way bricked if they've previously been coded to another car? Once coded, they cannot be re-coded, isn't that right?

That's right they can only be coded once.

[Paul-R]

Yes - the ID46 chips can but what about the remote locking? It's the imprecision of terms that I find exasperating.

[GiveMeABreak]

The ID46 chip is an identifier - once the BSI has written the code to it - that's it. This code now stored in the ID46 chip is used by the BSI to handshake - but not just the code - it is encrypted and 2 way handshaking takes place.

The use of the remote control functions also necessitates matching with the vehicle.

The coded message sent to the receiver (the box I referred to in the diagram (4)) by each remote control is composed of the following codes:

• A fixed code linked to the key
• A rolling code matched with the vehicle’s receiver
• A code corresponding to the action to be performed (Lock, Unlock, Switch Headlights on, Open Motorised Boot, Close all Windows etc.)
• A code corresponding to the condition of charge of the batteries in the remote fob

So the circuits in the fob still rely on the sending and handshaking of the correct codes from the transponder in the fob to the car's receiver unit before the BSI authorises the actions.

[Cutac]

I've coded the new key and the remote doesn't work. Again.
Since there are pre-set options for the third button, and since 3 button key has been working before, could it be that chinese keys are somehow not compatible with my car?
I honestly doubt it since my friend had ordered a key for his c5 X7 and it works. Not the same key/seller as mine, but still...

This is irritating, I want my remotely operated windows back!

[GiveMeABreak]

That is always going to be a problem with aftermarket ones. Pandora's Box - you just don't know what you are getting inside.

[Cutac]

Yeah...