Hello everyone, I drive an a6 c7.5 2.0 tdi quattro and I’m currently getting the code p0299( turbocharger underboost) when going above the speed of 50 mph and it puts me in limp mode. I’ve checked hoses for any cracks or leaks but cannot see any, my first bet was to try and either the replace the n57 valve or the actuator. Has anyone had any similar problems before I go into start replacing parts and hoping for a fix?

thank you for reading 😀

I am also having this same issue, same symptoms such as limp mode, same fault code. 

 

Have you managed to find what the issue is yet? I have cleaned the intake manifold and bought a few N75 valves. What I am struggling to understand is that the N75 Valve on a few I bought (and returned) was that none of them are ever holding a pressure when all the ports are closed off. Is this normal? 

Appreciate any help before I give up on this car

Edited October 11, 2025Oct 11 by kir4322

Dpf was blocked, dpf off and cleaned 3 times then put back on. All power is back and no limp mode yet! Tedious job so if not mechanically inclined I’d definitely take it to a garage.