Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
There has been a service bulletin for that type of concern. It involves a revised convertible top control module. This is what it states.
During opening sequence, tension bow has fallen out of range and top operation ceases. DTC 2000 (Switch position incorrect signal) will be stored in the Convertible Top Control Unit. This condition is more common when the vehicle is on an incline with the front of the vehicle elevated, such as when parked in a sloped driveway.
DTC 2000 will be the only fault present for this bulletin to apply. If there are any other faults in the convertible top control unit, these malfunctions must be corrected first. Only after these are corrected and the above condition is still present, accompanied by DTC 2000, should you continue.
Only technicians who have successfully completed Self Study Program 991203 - The Audi A4 Cabriolet and course number 991202 - Audi A4 Cabriolet Introduction are permitted to service the A4 Cabriolet. If you have not completed these prerequisites, do not continue with this bulletin. Have the case dispatched to a qualified A4 Cabriolet technician.
During the opening phase of the convertible top run, the front lock lifts from the windshield and the tension bow lifts to a vertical position to allow clearance for the convertible top box lid to open. The lid is then unlatched. During the unlatching of the top box lid and at the point when the tension bow is vertical, the opening strategy calls for the hydraulic pump to shut off momentarily. During this "off period" the tension bow is not under pressure and can slowly drift backwards before the top box lid can unlatch completely. Within this time period, the tension bow may have exceeded its plausible operating range. This will cause the top operation to cease. DTC 2000 (Switch position incorrect signal) is stored.
Optimized Convertible Top Operation Control Module beginning with software 9.0, part number 8H0 959 255B.
Sorry, I just got back from the doctor's office. There are a couple of possibilities on it. The module, (part number) or the top frame position sensor. The dealer would be the best bet for it, as the tests require special equipment and software.
Replace Convertible Top Frame Position Sensor G356 (8H0 959 613) with the improved version, which is identified with a yellow dot.