Motor Plan for Audi is worth nothing

My Audi is still under motor plan with 26 541Km on the clock. According to Audi the oxygen sensor is in default with engine light on. Audi Somerset West refuses to fix the problem, so did Audi in Clairemont Cape Town. They state that this part was not installed properly and that the vehicle must be fixed by the branch who initially (according to their records) worked on the vehicle. This means that the vehicle (now in Cape Town) must go back to Johannesburg for this fault to be repaired, if at all. It is evident that Audi refuses to take responsibility for items that must be fixed whilst under their motor plan - this is bad service and I will never again purchase an Audi. My previous vehicle was a Mercedes Benz and what a pleasure it was to deal with them when their vehicles are still under there motor plan - thus, prospective Audi buyers beware and be warned!!