1) Diagnosing faults in common rail diesel (CRD) systems requires testing both the low-pressure and high-pressure fuel circuits with a pressure tester to rule in or out issues. 2) It is important to first test the low-pressure side for faults before replacing high-pressure components, as low-pressure issues can affect the high-pressure side. Low-pressure tests check fuel pressure and look for blockages or restrictions. 3) High-pressure system tests measure rail pressure directly and check for faults in the high-pressure pump or sensor. Rail pressures can exceed 2000 bar and must be tested safely.