im googgling and im finding a lot of info about that fault code on a VW TDi
assuming they use the same fault codes it appears to be, and i quote.
the 17656 is telling you that your dynamic timing on your pump is out of range, when was the timing belt last done or has someone been playing about with the pump???
i hope this helps
PS i have no idea what that means, i just got bored so decided to do some digging for you
PPS
something else i just stumbled upon which makes more sense
The fault codes are indicating that your fuel pump could be on its way out, or something is interfering with fuel supply.
basic jist is from what im reading id your fuel pump could be cream crackered