help needed with fault codes

[scott_159]

i have a friend with a skoda octavia 1.9tdi 52 reg

now the car is just a basic model but very very slow

i beleiev he had the car on a diagnostics reader and these are the faults

p1156 short to ground presure sensor

p1556 turbo pres control

mass air flow readings low

can anyone please explain what these things are and where i can locate then on the engine

thanks!