1. Boom raised, bucket low speed digging and unloading, central bypass valve stuck in open position, oil output from main pump returned to hydraulic tank, moving arm rising and bucket working circuit inaccessible, central bypass line shaft removed for cleaning.

2. The moving arm rises slowly, hydraulic noise appears when the moving arm rises: the moving arm logic check valve is stuck for the reason that the pressure oil cannot enter the moving arm rising oil circuit, the moving arm logic check valve is removed.

3. Rocker extends or retracts at low speed: central bypass spool stuck in open position, oil output from main pump flows back to hydraulic tank, cannot enter bucket rod working circuit, central bypass dismantle valve for cleaning.

4. The motor starts to rotate, but the engine cannot be started: the central bypass spool is stuck in the closed position, the oil output from the main pumps 1 and 2 cannot return to the hydraulic tank, the central bypass spool is dismantled for cleaning.