Toyota Hilux D4-D Rev Problem

[devilsadvocate]

Hi, I work at a garage. Still learning stuff. Hope someone helps me on this so i can impress the boss. We have a Toyota Hilux D4-D 2KD engine. The problem is that when it reaches 2000 r.p.m, the check engine light turns on, the car does not rev anymore and there is a knocking sound from the engine. The car starts perfectly but as soon as it reaches 2000 r.p.m this problem occurs. The owner of the car went for diagnosis and it pointed to a faulty supply pump. we tried another supply pump but still no luck. Recently we changed the injectors but the problem still remains. Please guide me what else i can check. We have also changed the diesel filter and oil filter. Thanx