Short answer
A, B, C and D checks are graded inspections, from a routine A-check to a major D-check that may involve dismantling the engine. Which check your generator needs, and what it includes, depends on the engine model and the maintenance schedule in the maker’s manual.
A-check: routine inspection
Oil and coolant inspection Battery checks Belt and hose inspection Leakage checks General condition inspection
B-check: servicing
Oil replacement as per the maker’s schedule Filter replacement Fuel system and air filter inspection Cooling system inspection Electrical inspection
C-check: detailed inspection
Detailed engine inspection Fuel injection inspection Cooling system servicing Alternator and control system inspection Replacement of due components
D-check: major engine inspection
Major engine inspection, with engine dismantling where necessary Inspection of internal components, cylinder head and block Piston, liner and bearing inspection Reassembly and performance testing
Which one does my generator need?
The classification and the work in each check depend on the engine model and the applicable manual. Tell us the make, model, rating and running hours, and we will confirm which check is due and quote for it.
Written by the S. Y Diesel Services team. Published 2026-11-10. General guidance only: the engine maker’s manual and a site inspection decide the exact work for your generator.