Bad BCM, bad sensors, bad wiring or bad coding?!

tallpaul77

Active Member
Dec 15, 2022
28
9
Hi everyone, new to the forum after getting my 1.2 tsi FL MK3. Apologies for the long post

Been doing some basic coding with OBDEleven and had no issues - no fault codes on car

I came across a interior from an Xcellence ridiculously cheap with full door card and door wiring for ambient lighting.- no fault codes on car

To get things working like factory, I then realised I needed the BCM high l, so got a second hand unit - keys wouldn't work and few minor codes

Got the BCM coded at home and thought everything was working as should be. Drove the car next day and could hear the central locking clicking. Stopped and the rear offside door is not locking, boot is locked and won't unlock and the fuel cap is not locking.

Getting these four codes using OBDEleven

B12DC29 - Lock unit for central locking, rear, right Implausible signal

B12DC01 - Lock unit for central locking, rear, right Electrical error

B126301 - Tank lid unlock activation
Electrical error

B12DD15 - Rear lid Central Locking System Motor Open circuit/short circuit to B+

Is it just shear bad luck and ive got 3 things go wrong with the car at the sometime? Is it down to some issue with the BCM? Or is it down to the coding on the BCM?

Thanks for reading and any suggestions greatly appreciated!
 

mjj4

Active Member
Sep 30, 2019
97
76
It's definitely not going to be 3 central locking issues that have gone wrong at the same time. I'm not sure how much help I will be, but did you not have any issues with component protection on the new BCM? Did you take a log of the coding from your old BCM before replacing it?

The BCM high modules are used on several different models of Seats/VWs etc, so if it came from a 3 door, or golf etc... the coding would be different. I gather from what you've already written you are fairly confident with the coding aspect.

One thing I will say as I'm having current BCM issues with an implausible signal for the tail lights (meaning they are on all the time), the BCM doesn't appear the easiest thing to remove/replace, and the reason I'm having issues is because my rear washer pipe has split somewhere above the BCM, and when I tried to wash the rear window it soaked the BCM. This is obviously unlikely in your situation but worth making sure there are no faults in the BCM itself (Central Electrics 09). If that comes back clear I would have thought it would have to be something to do with the coding?
 

tallpaul77

Active Member
Dec 15, 2022
28
9
Thanks for coming back to me

I had the component protection removed and the new module coded to the car by a company. Keys were not working after the initial install of the BCM (before coding) and was unable to check if the issues were present. I'm currently debating installing the old BCM to see if the issues go, so I can then at least rule out wiring.

The fault codes mentioned in the first post are all from the BCM (Central Electrics 09). Faults will clear, but as soon as you lock the car, they come back
 

RUM4MO

Active Member
Jun 4, 2008
7,784
983
South Scotland
I'd think that you will need to remove CP from the original one if you re-fit and same again if you swop back to the new one.

Unless some "hooky" method of disabling CP was used.

Edit:- that above was offered as a warning just in case it does happen and you are left wondering "what next can go wrong!".
 

tallpaul77

Active Member
Dec 15, 2022
28
9
Thank you @RUM4MO I had it in my mind that both the original BCM and the new BCM would have the CP removed and that I could swap between the 2.

I guess next point of action is to test the wire continuity from the BCM to the boot lock, door etc

If thats ok, it just leaves a bad BCM or coding not correct. Using Notepad ++ I have compared the previous backup to the new backup and I can't spot any issues
 

mjj4

Active Member
Sep 30, 2019
97
76
From what I've been reading over the last couple of days about CP, you can swap the old unit back in, because the Can Gateway is the CP Master, and it sends out a request every time you turn the ignition on to all the CP modules for their CP data. So your Can Gateway obviously has not been modified by this BCM swap, and the old BCM will be original with its original CP data.

There is one other work around if that doesn't work, where you can unplug the BCM once the ignition is on and the Can Gateway has done its CP check, then plug the original BCM in, clear all faults and it will work regardless of its CP status. Only downside to this is whether the faults you are experiencing happen when locking and unlocking the car from inside with the ignition on.

It sounds like a coding issue to me, especially as you are using a different BCM, so certain bytes may not be the same, and I would assume copying your old coding over, then ticking boxes for things like ambient lighting will not work (if that makes sense).
 
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tallpaul77

Active Member
Dec 15, 2022
28
9
Thank you @mjj4

Guess no harm in trying to swap back to the old BCM. If the errors go, it must be down to the coding that has been carried out. Thankfully I actually need to pull the BCM for adding some additional wiring for retrofitting LED rear lights, but that's a whole different story!
 

tallpaul77

Active Member
Dec 15, 2022
28
9
Original BCM back in the car and all the issues have gone.

Considering other functions of the BCM work without issue, is it safe to assume the coding is not fully complete?
 
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