Your A2 has the later CAN BUS system. With the faults you have it is imperative that the first step is a full vehicle systems fault code read, print out the codes and then clear the codes. Then take the car for a test drive if possible and more than likely one of more codes will come back and so a re-read and print out those codes can be compared with the first read and we can take the next step. Some codes you can get may only come back after a quite lengthy drive along a mixture of roads, in particular, a long straight. There are certain fundamental errors you could have made involving centring the rack, it is extremely important that the rack is precisely centred at 72.5-mm (dimension A) with the subframe aligned with special locking pins and with the steering wheel in the dead centre position (only possible by using the Measuring Blocks function of the ABS system on a diagnostic display in Display Group 004, Zone 1) only then should the column be connected to the steering box. Next the tracking must be set so that the wheels are pointing directly straight ahead at zero degrees; the setting is a very precise and slightly odd 4′ ± 5′. It is important that the front wheel camber of - 35′ ± 20′ is correct BEFORE setting the tracking. Then you have to perform the Basic Settings procedure by logging into the ABS system using code 40168 and then perform the zero compensation setting of the steering angle sensor in Basic Settings 04, Group 060. When this is done, hopefully you will have no further problems but if you do, you will have to work through the fault codes as they come up until the system is happy. The other potential problem is of course the rack you have fitted. There are four racks used on UK spec A2's from 2000 to 2004 and the one for the 2004/5 models is specific only to cars of the 2004/2005 model years so cars from VIN WAUZZZ8Z14N000001 onwards.