Boost air pressure low

Air manifold fouled and leaking or exhaust gas leaking between engine manifold and turbocharger inlet.
Pressure gauge reading incorrectly or connection leaking.
Dirty air filter / rotor assembly / diffuser – nozzle. Labyrinth seals worn or dirty.
Turbine / nozzle blading damaged.
Exhaust back pressure too high.
Compressor wheel fouled.
Defective bearings.
Rotor touching or foreign object restrictions.