Wrong Diagnosis

On the 8th February 2016 I took my Audi Q5 to Audi Menlyn branch as it was having gearbox malfunctioning message on the dashboard, then they fix it and I paid R15,000,00 then after 2 weeks it gave the same problem and I took it back as the parts were still under warranty. Then they told me that they can't fix it, after 4 days they told me that now it shows them another problem which I need to pay extra R30,000 on top of R15,000 that I have already paid 2 weeks ago. To me that means they did the wrong diagnosis on the first time cause even after replacing those parts it continues showing same message/same problem if Gearbox malfunctioning, Audi Menlyn is responsible for fixing my car, they must stop taking their clients for a ride, to me customer service and competence is non existence in that dealership, they are very irresponsible and unprofessional. Now my car is stuck in their garage cause I cannot pay another money because they did a wrong diagnosis work, I'm prepared to add on what I've already paid, not for them to tell me that after paying R15,000 I still need to pay another R30,000+ on top