Citroen C3 Fob problem

[epipete]

Given the nature of the LEC Forum and this particular thread I thought maybe someone might shed some light on this problem:

At midnight the other day my lad, the proud owner of a Citroen C3 (passed his test a fortnight ago) phoned home and advised 'I may have a problem', the crux being that he had parked his car earlier and locked it up and all was fine, on returning several hours later his remote key wouldn't unlock the doors, manually opening the doors allowed him access but the key would not start the car (the starter fired but no ignition). Eventually the battery failed and he called home!

I tootled out with jump cables (but not the spare remote). On arrival we jumped the battery but still could not start it. I assumed it was the immobiliser but couldn't fathom out why. Changed the battery in the fob. Eventually we called out the AA meantime I went home to pick up the spare key - a 45 minute journey each way!!! By the time I got back the AA man had arrived, and somehow started the car. The Spare key that I returned with worked perfectly - opening the doors and allowing ignition.

So now he has the car back on the road with one good key and one u/s. But why? actually there are several questions:

What has happened to the original fob to stop it working?
Why do both facilities fail - door locking/unlocking & immobiliser (I thought the electronics and immobiliser chip were independent of one another)
How do I test the old fob, and can it be repaired?

Any thoughts, please.

Pete

[Tuga2112]

Hi Pete,

Here's my good intentioned 2 cents to the topic. Hope you find some of the information helpfull

I can tell you from experience back long time ago in Portugal (we are talking about 10 -15 years ago) when my grampa bough a volvo (maybe a 960 not sure) the car would immobilize if the fob was not used to unlock the doors. it was a feature of the car.
I am not surprised to find the C3 to rely on the fob being used in order to have an added immobilization step to the car.

As you know I'm nowhere near an electronics expert. but i would open up the fob just to check if there is any burnt circuit. something physically damaged should be visible by the naked eye. (and when i couldn't see anything wrong feel ignorant.. in fact i would feel ignorant either way because wouldn't have any idea how to fix it.)

Is it repairable? probably not the Fob itself, I think the problem shouldn't be difficult to solve with a trip to a Citroen dealer, but it may be cheaper to go to a keycut shop and make a copy of the working fob, nowadays they usually are capable of making fobs, after all they can make keys with the immobilizer codes.

[Ian T]

Pete, I can't say anything specific to the C3, as I don't know how integrated the central locking is to the immobiliser. Sometimes it is, sometimes (the majority of the time) not.

As for the remote not working, if there's a chance the button has been pressed a few tens of times outside the range of the car, the rolling code will have advanced on the fob but not on the car and therefore it will not be recognised as an 'expected' code when he returned to the car. The look ahead number of expected codes in the sequence is quite large, often as many as 50 presses of the button, but is nonetheless not infinite. You may remember the rolling code being invented to prevent thieves from scanning the fob code over the air and replaying it to unlock the doors. Similarly if the battery is flat for a while, or removed for a couple of minutes, the current position in the code is forgotten and the car doesn't know, so it needs to be re-learned, or 're-synchronised'.

If this theory is correct, it might be worth googling 'resync C3 fob' or something similar, resynchronise, program, etc. The process is usually really easy and for many cars is simply turning the ignition on and then pressing the fob buttons until the locks cycle. That process works for Vauxhalls and some others.

Hope this helps - I'dsay it's the most probablee xplanation.

Ian.

[epipete]

Thanks guys, it seems that the key fob contains an immobiliser chip and a central locking circuit and that they act independently of one another; hence if I change the good circuit board between the two fobs, each with their individual chips, then the engine will start with both. This tell me that it is the circuit board that is kaput!

I've tried to reprogram the key fob without joy. Any ideas of a simple way of determining if the broken board us transmitting a signal?

[GeoffSmith]

Some light background reading. The switches can be prone to damage but are replaceable if you know the right person.

[epipete]

Thanks Geoff, much appreciated.

I took the fob off the hook this afternoon with me to Simpsons and asked them how to confirm if it was transmitting a signal, the guy brought out a machine pressed some buttons and said not! When I got home I realised that the key he had tested was the working one and, yes, I was able to open and start the car with it! So much for Simpsons!!!

I'lol have another look at the circuit board and see if there are any obvious faults

Pete