A few weeks ago the airbag warning light in my 2004 Audi A3 2.0 TDI came on. I took it to an independent Audi specialist for diagnosis, hoping it would just be a sensor or something else minor, as the garage thought it would be. The diagnosis came back with a fault code saying I had a faulty airbag ECU, which the garage said was definitely correct but also extremely rare. I bought the unit brand new directly from Audi, and had the independent specialist fit it at a total cost of over £400.

All was fine until two weeks later, when the light came on again. I took it back to the same garage and they pulled up exactly the same fault code, and told me that the new unit was also faulty and directed me to an Audi main dealer, as I had bought the part from Audi. Audi said how hugely unusual it was for one unit to fail in a car's lifetime, let alone two, but as they had come up with the same diagnosis they nonetheless replaced the part under their parts warranty at no cost to me.

Surprise, surprise, a few weeks later and the same light is on again with the same fault code. The original fault has clearly been misdiagnosed, but who do I complain to? The independent for misdiagnosing in the first place, or the Audi main dealer for also misdiagnosing? And what is the best way to go about it, seeing as the original bill came to over £400 and it looks very much like it was completely unnecessary.