A knock sensor problem. Why You SHOULD NOT Buy A 2017 Audi Q7! Watch on. Often, Audi Q7 owners will drive down the road and find their check engine light is on and in the solid illumination state. No performance issues noted, but sometimes the knock (NOX) sensor light will show up on the dash.
Im surprised you get 39 mpg with the 3.0 TDI as i only get 40 mpg with the 2.0 TDI 170 Quattro. This is mostly motorway miles and i dont dry with the foot down all the time either. There are threads on here discussing MPG for the 2.0 TDI 170 Quattro and I haven't seen many registering MPG above 40-45.
There's a reason half the cars in Boulder are Audi's. It's been reliable transportation and lovingly cared for. Rating breakdown (out of 5): Comfort 5.0; Interior 5.0; Performance 5.0;UjsT.