Awesome GTI tuning this week!
Hi Mac I think margin-walker means pollen filter. It's located under the bonnet on the nearside under the windscreen scuttle cover. Just unclips so easy to replace. Worth checking the airflow here first, leaves can block too. Check your fuses too. The dash display may not show a light even with a fault on the aircon system, so a plug in diagnostic check will be needed. Good luck with fixing it and please let us know the solution you found.
Audi A4 B7 2004 red 2.0 TFSI Quattro Sport saloon, Servotronic steering, MFSW, CDIS, DRL with Coming Home, auto dual xenon lights/wipers/mirrors, split folding rear seats, 3rd headrest, centre armrest, Isofix mounts, RS4 rear ARB, pedals & carbon dash trims; Audi TTS 9Jx18alloys (2 sets), Yokohama AD08 summer, Conti TS830P winter 245/40/18 tyres, TPI 15/20 spacers/lockers, Pipercross filter, GFB DV+ turbo valve, NGK BKR7EIX plugs, Osram Silverstar Xenarc D1S bulbs. Build thread