Ibiza cupra 1.4 gearbox

[Michael w]

Hi everyone new to this forum , I purchased my 2010 seat cupra Ibiza about 6 weeks or so ago . Going by the service history the dealer had the mechtronic replaced before it sold it . Been fine until yesterday when it was struggling from going from 2nd to 3rd if it eventually got up to 4th it was fine going through higher gears . But today it struggled and the gears are flashing d1 etc and it’s missing d3 altogether and still flashing.
I’ve run it through vag com and came up with the code

Position sensor 1 gear selector
P173a

engine management lights on now .
Any ideas what this could be , is it the mechtronic or gearbox or clutch ?
If it’s gearbox will a rebuild fix this problem ?

thanks in advance for your help