The VAG Group 2.0 TDI engine found in this Audi A4 is common for EGR Valve faults we have a number or options from replacing the EGR to Removing the EGR functions. The replacement EGR is expensive and can fail again in the future. The best option is to disable the EGR valve so effectively prevent the actual valve from moving or changing position.
The codes most common with EGR failures are P0400 and P0401, Limp mode is almost certain with either P0401 and P0400 you may even have both fault codes.
We are able to help with either a new EGR or an EGR Delete. Please feel free to get in touch with us, Our workshop is based in Saxmundham suffolk. We are situated in between Ipswich and Lowestoft on the A12.