Hi

I thought I would just share my experience I had of turbo problems with my A6 2014 2.0 TDI Ultra, in case others have the same issues
So what I was happening was the engine fault light was coming on, Coil light was on, and car went into limp/safe mode. This happened all the time, usually when the car was pushed (going up a hill, or accelerating), and it would go into safe mode. It would also occasionally misfire.

Engine fault codes that came up:
P023600 - Turbo charger boost sensor "A" circuit range
P023400 - Turbo Super charger overboost condition
P023800 - Turbocharger boost sensor "A" circuit high input
U111300 - Function limitation due to received malfunction value
P227900 - Intake air system leak
P13D600 - Sensor for internal pressure of cylinder 3 implausible signal

So, I did a bit of looking around, saw that some people had replaced their turbos, but it didnt solve anything. So I went to my local mechanic and told him to replace the turbo pressure sensor (S04L906051K). What a difference! Car back to its old self, loads of acceleration and power, no engine fault codes/coil warning lights!