I had this issue pop up yesterday afternoon with my 66 plate Audi A4 B9 Avant 2.0 TDI. Called Audi Assist and was given the option of RAC in 30min or wait for Audi. Was towards the end of the day so booked for Audi to come first thing this morning. Turned up before 9am and the guy said straight away he knew what the issue was and replaced the sensor. Was here for 25minutes in total.
Cheers all for the advice in this thread, likely would've been into the garage for £100s (at least) otherwise! Worth getting Audi Assist out first if you can.
