Audi a3 2009 1.6 petrol loss of power in 2th gear?

CHANGE the FUEL FILTER! I once worked on a fifteen year old VW BEETLE convertible! The CHECK ENGINE light cm eon and the dealer said it had a BAD TRANS for about $7.000>>>>I FIXED IT by cleaning the THROTTLE BODY and PCV system and replacing the FUEL FILTER! The TRANS was FINE and the light NEVER came back on again!>>>INI YOUR CASE< i would get a FULL TRANS SERVICE DONE and change the fluid using the FACTORY PENTOSIN FLUID! So GO the DEALERSHIP! While you are THERE< if your brake fluid is VERY DARK BROWN, change it! AUDI has updated ALL the fluids to make things as PERFECT as possible! SO CHANGE ALL THE FLUIDS! The NEWEST coolant is VIOLET/LILAC in color! The NEW chf 202 STEERING fluid is AQUA BLUE! uSE only 502/505 pure synthetic oil LIKE Pennzoil ultra euro TO GET A WARRANTY TO 500,000 miles!  IF you EVER used VALVOINE in that thing you NMAY  need all your PCV HOSES and VALVES REPALCED! VALVOLINE is JUST NO GOOD and ROTS hoses and seals and gaskets! It is MADE from ASH and WAX and returns to ASH and WAX! IT can cost you an ENGINE in the long run! Use BLAUPARTS< 1-800 ALL EURO and 1-800 ALL AUDI for more answers! OTHER OILS to use are TOTAL ENERGY OIL from FRANCE< AMSOIL for  EUROPEAN cars ( good for 25,000 miles with ONE filter midway) LIQUI MOLY, RAVENOL from Germany. LIQUI MOLY makes DIESEL PURGE ! It is the BEST thing out there! It will CLEAN the injectors and the diesel pump, like a TUNE UP in a CAN!   

Check for a bad transmission fork or fork bushing.

What are the symptoms? Maybe slipping clutch? It's possible the gearbox has a fault, but unlikely.