Any idea what could cause this? VCDS scan below. To me it looks like something is preventing communication on the can-bus, but what could it be? All modules are detected, all soft coding is correct. Have verified ground integrity at the modules, battery voltage is good, car runs well.

Not that it isn't clear in the scan, but car is a 2001.5 s4 with esp/abs.

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Address 01: Engine        Labels: 078-907-551-AZA.lbl
   Part No: 8D0 907 551 M
   Component: 2.7l V6/5VT     G   0002  
   Coding: 06711
   Shop #: WSC 00001  
   VCID: 3469CA6D04B2FBB8F54

3 Faults Found:
18057 - Powertrain Data Bus
            P1649 - 35-00 - Missing Message from ABS Controller
17931 - Crash Signal from Airbag Controller
            P1523 - 35-00 - Implausible Signal
18058 - Powertrain Data Bus
            P1650 - 35-00 - Missing Message from Instrument Cluster
Readiness: 0110 0100

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Address 03: ABS Brakes        Labels: 8D0-907-389.lbl
   Part No: 8D0 907 389 E
   Component: ABS/ESP allrad      D36  
   Coding: 04395
   Shop #: WSC 06435  
   VCID: 356FF7690FC8E2B0C2A

2 Faults Found:
01203 - Electrical Connection between ABS and Instrument Cluster
            03-10 - No Signal - Intermittent
18258 - Powertrain Data Bus
            P1850 - 35-00 - Missing Message from ECU

Is the bus termination ok? What impedance are you getting between the high and low cables?

Resolution (because I hate when people don't update their threads)

The large red plug under the ecu was half out. All sorted now.

Your issue is the ecu. Methinks it uses a different protocol to communicate between the modules in the car.