If it helps, this is the one mechanical problem my A6 had, and it just needed a new G450 sensor fitting and calibrating. If you have VCDS you can see the ash content in the DPF and the last time a regen happened, which might give you a good idea of whether it's just a failed sensor or a blocked DPF.
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MY2012 A6 Saloon 2.0TDi S line - Ice Silver, Tech Pack, DAB, Storage Pack, Lighting Pack, Heated F+R Seats, 4 zone A/C, 75l Tank, 20" Rotors.
Ordered: A6 Saloon BE Ultra S tronic - Floret Silver, Tech Pack Advanced, Parking Pack Advanced, Heated F+R Seats & Wheel, Electric Seats, Matrix LED, Ambient Lighting, Speed Limit Display, Twin Spoke 20", Folding Memory Mirrors.