Solved: C5 immobilizer forgot the key?

antonzlo · Unread post by **antonzlo** » 24 Jul 2023, 12:00

Hi all,
I know there is plenty of topics about immobilizer on this forum, but my situation is a little different.
I don't have "immobilizer fault" message, the car just doesn't start normally. It does, if you crank with dead battery, maybe some module is shutting down and not stopping the engine.
So, engine is working, but immobilizer is preventing it from running.
May it be so, that it forgot the key? Or the RFID chip in the key suddenly died?
My friend with launch has read the car, there were no relevant faults apart "engine ecu communication continuous fault" in airbag module and he wasn't able to speak to ecu. Is immobilizer just disabling the whole ecu?

Upd: it was fuel level sensor, the car just ran out of fuel.

GiveMeABreak · Unread post by **GiveMeABreak** » 24 Jul 2023, 12:19

I'm not really understanding fully the issue, but yes the Engine ECU will not unlock the engine ECU if the BSI is not getting the correct handshake from the transponder in the chip.

The Chip in the key or fob has to respond to the request from the BSI, which then passes this onto the engine ECU and backwards. If it isn't matching up then the engine ECU won't allow the vehicle to start.

1) Make sure your vehicle main battery is healthy and fully charged
2) Follow the part in this procedure to reinitialise the fob if the battery has been disconnected:

BSI Reset / Battery Disconnect / Reconnect Procedure

antonzlo · Unread post by **antonzlo** » 24 Jul 2023, 12:33

Thanks for the reply, my battery is new and now charging, I'll reconnect it and try reinit.
The full issue is I was driving yesterday, and the car suddenly lost power along with "esp malfunction", but I think it's just a parasitic message. I wasn't able to start it for longer than 2-3 sec.
There is no message when starting, just doesn't start. It tries on the first cycle, but then is cut out.

GiveMeABreak · Unread post by **GiveMeABreak** » 24 Jul 2023, 12:45

The starter will turn over, but will not start the engine if the Engine ECU is locked.

antonzlo · Unread post by **antonzlo** » 24 Jul 2023, 12:52

So, I completed the reinit procedure, remote control works, but the car still doesn't start.

GiveMeABreak · Unread post by **GiveMeABreak** » 24 Jul 2023, 14:00

Then you have another issue if you are sure you have reinitialised the fobs. It needs a diagnostic check with a Lexia as hid scanner may not be able to communicate with all vehicle systems.

antonzlo · Unread post by **antonzlo** » 24 Jul 2023, 14:07

Okay. Does Diagbox V7.83 work?

GiveMeABreak · Unread post by **GiveMeABreak** » 24 Jul 2023, 14:22

Overkill for a MK I C5, but yes, it will launch Lexia automatically. Your VCI must be a full Chip version revision C to read CAN vehicles like the C5.

Ddclutch · Unread post by **Ddclutch** » 24 Jul 2023, 19:05

Just a comment, but many cars will start but then immediately shut down again after a few seconds if the transponder chip is missing or damaged. It can happen if you drop the keys. It can seem confusing that the car should start at all, but that is what happens.

antonzlo · Unread post by **antonzlo** » 24 Jul 2023, 19:09

The weird thing is that I managed to drive about 10 minutes before fault. I've opened a key, the sticker seems intact. Battery is charged to 3v. But no change in behavior if I remove circuit board from the key and try to start, or if I disconnect the coil on car side. I was hoping for immobilizer fault to show up.

Unread post by **Rp0thejester** » 24 Jul 2023, 19:44

It seems everything is aimed at the key at the moment. If it has started and worked then I would look elsewhere as well. Fuel sounds a massive alarm in my head. Battery is good, starter motor is good as it's cranking, a signal is getting there but is the fuel??

antonzlo · Unread post by **antonzlo** » 24 Jul 2023, 20:28

Rp0thejester wrote: 24 Jul 2023, 19:44 a signal is getting there but is the fuel??

Sorry that didn't mention earlier, there is fuel pressure, we checked via fuel rail "tyre pump" valve.
And, when the battery went flat from our attempts, some block probably switched off for a moment which allowed the engine to start and run for 2-3 sec, before getting cut off.

Unread post by **Rp0thejester** » 24 Jul 2023, 21:32

I still feel fuel is an issue, if it's cranking as in starter motor is trying to start then the key has recognised itself and letting the electronics get to the starter motor. So I would be definitely checking fuel filters and lines

antonzlo · Unread post by **antonzlo** » 24 Jul 2023, 21:56

From what I read in the other topics ECU cuts out spark, not fuel
+ when trying without key's pcb it was cranking still.

Unread post by **PaulC5** » 24 Jul 2023, 22:30

I think you can check if the key is being recognised by the immobiliser by putting the key in the ignition and then opening the driver's door. If recognised on most cars you get a warning to show the key is in the ignition and not removed. If you do not get a warning then the key is not being recognised. Something else to check is if the car is stuck in eco mode.

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Solved: C5 immobilizer forgot the key?

Solved: C5 immobilizer forgot the key?

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