I have a omege cdi and had the water pump and cam belt done, crank sensor was change for a non genuine part.

air bag light and eml(engine with spanner) light came on and stays on, rev counter and speedo drop and engine stutters, also the air con flickers and blows mad only last a split second then back to normal. warning light still on.
went back and had gen crank sensor fitted also a new alternator as supected that was the fault, (low voltage).
but to no availe still playing up and vauxhall want 400 quid to try to diagnose the fault.
anyone have an idea as to what the ********** is happening?