Diagnostic Overview
Cummins fault code 559 commonly indicates that the fuel system cannot maintain commanded fuel rail pressure. The ECM may see actual rail pressure lower than expected under load or acceleration. Common causes include plugged fuel filters, low supply pressure, air in fuel, weak lift pump, high-pressure pump wear, fuel pressure relief valve leakage, injector return leakage, pressure control valve problems, or a faulty rail pressure sensor. Diagnosis should include low-side supply pressure, filter condition, commanded versus actual rail pressure, injector return flow, and pressure relief valve checks.