Fuel system topology
Modern light diesels are common-rail high-pressure systems running 25,000 to 35,000 psi. Know the difference between a CP3, CP4, and CP4.2 pump, why the CP4.2 has a well-documented failure mode, and what happens to injectors and rails when a pump grenades. A9 will ask you the correct repair scope after a pump failure.
DPF, DEF, and SCR
Diesel Particulate Filter regeneration, Diesel Exhaust Fluid quality, and Selective Catalytic Reduction NOx control together account for a huge share of A9 questions. Know why a stuck-open EGR causes low regen temperatures, why bad DEF (contaminated or expired) throws NOx efficiency codes, and why disabling any of it is a federal offense.
Glow plugs and starting
Cold-start questions still show up. Know how a modern glow plug controller monitors individual plug current and sets a specific glow plug code, and why parallel wiring means one bad plug does not disable the others but does affect cold start quality.