Hiya, I have a beloved AUDI A4 TDI 1.9 (B6 96KW 2001) The dashboard Check Engine/MIL/ECU light is not turning on when it should be. They have made this light a requirement for UK MOT tests this year (oh gawd). Main Issue: • No ECU light on and off when ignition • No ECU light when engine sensors are deliberately tampered with (MAF) Had a diagnostic recently done: • Car engine is running fine • ECU communicates with diagnostic tool • ECU logs any fault codes correctly (e.g. MAF tampering) • I thus assume the ECU work okay (engine running fine) • Cluster ECU Check engine light was tested OK via diagnostic tool • I assume the cluster is okay as the bulb can be switched on and off How would I start to get someone to diagnose this and fix the issue? I understand the immobiliser network has three elements - the ignition, ECU and dash cluster? If each of the elements in this group is working individually, should I conclude there something broken between the ECU to dash cluster which is preventing the light from turning on and off for ignition (or when the engine sensors are deliberately messed with)?? Could it be a wire, a seating, a pin connector? How can I diagnose this issue and where would be the right area of focus? Welcome any advice or questions.