Diagnostic Overview
Mack fault code P0087-00 indicates low fuel rail or fuel system pressure. Although listed as a derate-related code, it is usually a fuel delivery fault that can cause low power, hard starting, misfire, engine shutdown, or torque reduction. The ECM is seeing that actual fuel pressure is below the commanded value.
Likely Causes
Restricted fuel filters; weak supply pump; air in fuel; suction restriction; contaminated fuel; pressure regulator issue; high-pressure pump wear; injector return leakage.
What To Check First
Start with fuel filters, fuel quality, supply pressure, and air-in-fuel checks. Compare commanded versus actual rail pressure under load.