Diagnostic Overview
Mack fault code P009B13 generally relates to the fuel pressure relief control circuit or fuel pressure control system. A circuit or calibration-type fault can prevent the engine from controlling fuel pressure correctly, which may trigger low power or derate protection. This should be diagnosed before replacing expensive fuel-system components.
Likely Causes
Fuel pressure relief valve/control fault; open circuit; wiring damage; poor connector contact; pressure control component failure; ECM control issue.
What To Check First
Inspect the fuel pressure control wiring and connectors, verify circuit integrity, and compare fuel pressure command versus actual pressure.