Where to start to fic error code p1125? Egr already cleaned.

Question

Engine light is on. Cleaned soot from egr, which improved running slightly, but engine light was still on. Now it's intermittently blowing thick grey/black smoke and running very rough, unresponsive to excelleration, not revving. Etc.
Engine does not cut out or shudder on ignition kill.
Will try cleaning throttle body and connections within it on p.circuit board, and check swirl flap actuator, (and arm, but where is it?), but have now read that the (62) abd 0400 codes relate to boost pressure solenoid valve or vaccuum pump ???
Any info or tips about what to check first (and how) would be much appreciated.