C4 Replacement Fob Not Working

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Re: C4 Replacement Fob Not Working

Post by Cutac »

I was about to order this when I saw it was ASK.
https://www.aliexpress.com/store/produc ... autifyAB=0
When looking for 3 button fobs with FSK modulation, all PCB's look worse than the linked one and almost none of them have micro switches.
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Re: C4 Replacement Fob Not Working

Post by Cutac »

Can a non cut and immobiliser-non-paired fob be set up for remote unlocking/be synchronised?
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Re: C4 Replacement Fob Not Working

Post by GiveMeABreak »

The key blade is irrelevant apart from unlocking the ignition barrel and turning the engine over.

A fob, of the right type, once coded to the vehicle, will both operate the central locking and any other function where applicable. It can only be coded to one vehicle though.
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Re: C4 Replacement Fob Not Working

Post by Cutac »

So, the fob has to be coded, otherwise it can't operate the remote locking?

I was wondering about operating the remote locking and unlocking by an non-coded fob.

Switching the ignition on with an already coded key, but then holding the lock button on the non-coded key(brand new) so it would operate the remote locking. Possible?
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Re: C4 Replacement Fob Not Working

Post by Paul-R »

That was also a theory I had for my Mk1 C5 where the ID46 chip is a separate item from the remote unlock board but unfortunately I never got around to try it before it went. When someone else on this forum tried a similar experiment they said that it didn't work.

I have also bought an extra fob for my X7 which arrived a few days ago and I had the blade cut today. I tried it in the key barrel to check it was cut correctly and then wondered whether I should try to pair up the remote locking. I decided against it on the grounds that everything is on the PCB and if I screw it up then I've lost everything achieved so far.
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Re: C4 Replacement Fob Not Working

Post by GiveMeABreak »

Cutac wrote: 05 Mar 2019, 22:56 So, the fob has to be coded, otherwise it can't operate the remote locking?

I was wondering about operating the remote locking and unlocking by an non-coded fob.

Switching the ignition on with an already coded key, but then holding the lock button on the non-coded key(brand new) so it would operate the remote locking. Possible?
That won’t work! Otherwise anyone could buy a blank fob and get in your car if it did not need to be coded.

The remote locking unlocking function also uses the transponder to send the correct code to the high frequency aerial and electronic box in the vehicle which allows the BSI to authorise unlocking. It therefore has to be coded before the electronic box will accept the request.
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Re: C4 Replacement Fob Not Working

Post by Cutac »

Alright, got it!

My idea was somewhat similar to Paul's.
I've got two different generation Citroen's, and my plan was to try the fob on the B7 and if wouldn't work, try it on the VTS. Now if I try it on the B7, thus cutting and coding it, if it doesn't work, I won't be able to use it on the VTS.

Now that I think of it, it wouldn't make any sense for the fob to be able to be paired with a car it hasn't been coded to. That way someone could lurk at the parking lot, wait for someone to put their key in the ignition, and code his blank key to their car... :mrgreen:
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Re: C4 Replacement Fob Not Working

Post by Paul-R »

GiveMeABreak wrote: 05 Mar 2019, 23:42That won’t work! Otherwise anyone could buy a blank fob and get in your car if it did not need to be coded.

The remote locking unlocking function also uses the transponder to send the correct code to the high frequency aerial and electronic box in the vehicle which allows the BSI to authorise unlocking. It therefore has to be coded before the electronic box will accept the request.
That's not the theory behind it. You still need a working key for what I wanted to do.

I theorised that if you put the working key in the barrel and then turn to the first position you could hold the other key fob near to the barrel and do the pairing with the second fob.

To be honest, I still don't know why it doesn't work. If you think about the earlier keys with a separate ID46 chip some people have taped the chip to the ignition barrel and are effectively doing exactly what I'm suggesting with separate fob.
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Re: C4 Replacement Fob Not Working

Post by GiveMeABreak »

Of course it won't work, that's not how it's paired (coded).

That will only work with a fob that has already been coded to the vehicle.

There are 2 issues here - coding a new fob (done once) and re-synchronising which may be required periodically.

A new fob that has not been coded using Lexia will do diddly squat - which is what Cutac was talking about.
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Re: C4 Replacement Fob Not Working

Post by Paul-R »

No, you're missing my point. There IS a coded key there in the barrel It's only the ID46 chip which is important at that point, not the remote locking. Bringing a different fob up to the barrel and performing the synching procedure should work. I'm not saying there will be two fobs linked, just the one you've synched.
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Re: C4 Replacement Fob Not Working

Post by GiveMeABreak »

No - you are missing the point Paul. That is NOT how it works. :chin:

To get the Transponder chip coded in the first place it must be coded with lexia using the confidential code to programme the fob. Once the Transponder chip is then uniquely coded, it is this that is energised by the ignition transponder coil and the signals exchanged using encrypted rolling codes.

You cannot use a coded transponder chip / or fob to code another blank fob / code - not possible to programme it doing that which is why it will never work.

Say you had a key with no fob, and a separate fob with no key; as long as the fob had been previously coded, and it was stuck to or in close proximity to the ignition transponder, it should allow the car to start. But if I understand you correctly, you are trying to say if you had a working fob and key in the ignition and stuck another blank non coded fob next to the working fob, you are hoping to code the blank fob? If this is the case, no, it won't work. :-D
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Re: C4 Replacement Fob Not Working

Post by Paul-R »

Are you sure of that Marc? I've never seen that written anywhere before. As I understand it the confidential code is used to pair the ID46 chip. Then when that's done you pair the fob using the synching process. You're saying you need the confidential code to synch? Never needed it to resynch before. At what point do you need it, and how, do you input it during a first synch?
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Re: C4 Replacement Fob Not Working

Post by Paul-R »

GiveMeABreak wrote: 06 Mar 2019, 09:59To get the Transponder chip coded in the first place it must be coded with lexia using the confidential code to programme the fob. Once the Transponder chip is then uniquely coded, it is this that is energised by the ignition transponder coil and the signals exchanged using encrypted rolling codes.
Not disagreeing with that at all.
GiveMeABreak wrote: 06 Mar 2019, 09:59You cannot use a coded transponder chip / or fob to code another blank fob / code - not possible to programme it doing that which is why it will never work.

You're saying chip and fob interchangeably. Certainly in the early keys (Mk1 C5. Phase II Xsara), they're separate items.

One of the keys for the Xsara didn't have a remote unlocking fob. It sounds like you're saying I couldn't have synched a remote fob to it?
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Re: C4 Replacement Fob Not Working

Post by Paul-R »

GiveMeABreak wrote: 06 Mar 2019, 09:59Say you had a key with no fob, and a separate fob with no key; as long as the fob had been previously coded, and it was stuck to or in close proximity to the ignition transponder, it should allow the car to start. But if I understand you correctly, you are trying to say if you had a working fob and key in the ignition and stuck another blank non coded fob next to the working fob, you are hoping to code the blank fob? If this is the case, no, it won't work. :-D
Should have consolidated all my answers.

Almost. I'm saying that you should be able to synch the separate fob. You'd probably lose the synch to the built-in fob though.
As I get older I think a lot about the hereafter - I go into a room and then wonder what I'm here after.

Inside every old person is a young person wondering what the hell happened.

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Re: C4 Replacement Fob Not Working

Post by Cutac »

GiveMeABreak wrote: 06 Mar 2019, 09:59 you are trying to say if you had a working fob and key in the ignition and stuck another blank non coded fob next to the working fob, you are hoping to code the blank fob? If this is the case, no, it won't work. :-D

I'm not sure that's what he meant. Or that's what I meant. This is my thought process on this:
You put a coded fob(Gary, stay with me:mrgreen:) in the ignition, twist it, car starts.
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) .
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.

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