No cluster, no radio, no phone and fuses good

120k miles on A3 TDI 2.0 2012
After changing a totally dead battery I ended up with a car that starts but the immobilizer kicks in within the second.
Checked all the fuses in both boxes and they are good.
No working cluster, no radio and no phone made me check if there's 12 Volt on the fuses and there's not.
No power to the cluster means, the immobilizer doesn't recognize the key. All within the logic of the design.
Since I don't have an electric schematic nor have much experience trouble shooting the A3, I'm at a bit at a loss.
Either the power to the cluster/radio/phone fuses comes from a relay or switch or there's a fuse hidden somewhere that is not part of either fuse box.
Loose connection is possible, I didn't remove the cluster yet to look behind to see what's there, wanted to see if anyone on this forum had an inkling of where to look further.
Any secret reset sequence I have to go through after a battery replacement ? We were able to jump start the car prior to the battery change.
Yep, I need to drive this car on its own power to the dealer for the buyback ...
Any suggestions much appreciated ! Thanks in advance,
Rudi Blondia

It goes without saying that the dash, radio nor interior light activate when any of the front doors are opened.

My local dealer doesn't want to service the car unless it drives in under its own power since it will be a buyback as soon as Audi has all the paperwork together ... applied the first in November and this is still not resolved in spite of weekly calls to the customer service number.

Great car but disappointed by the current state of affairs ...

Rudi.

maybe there is some sort of immobilizer code you need to enter? When i had my honda accord, there was radio code to enter after you replace the battery.

unless you have been removing the cluster often the likelihood of a loose connection there is nil.

have you tried other keys?

get a VCDS cable and see if you can run a scan with the key on and doors unlocked.

Tried with both keys, no difference

All I know for sure there's no 12 volt on the fuses for the cluster, radio and phone. All other fuses get their power.

The relay (100) in the fuse box under the hood is good. Will be working on this again tomorrow and see if I can get a VCDS scan or find the relay that controls the power to the above fuses.

Finally got time to get back to the Audi A3, was a busy workweek.

Checked out all the relays under the dashboard and the lone fuse sitting there. all good.

Did one last check of the fuses under the hood, next to the battery box, but this time testing the fuses when conducting current.

Bingo, the small body 40A fuse upper right in the box is intermittent. No visible burn mark or open wire but depending on the stress on the terminals, I got an intermittent contact.

Replaced the fellow with a spare one and all started up as normal. Bunch of error codes but no surprise as the battery was disconnected a few times during this exercise.

Driving it tomorrow to work and hoping it all clears.

Thanks for reading and responding !