PACCAR MX (2004 Emissions) Fault Code: P109C Common Rail Pressure Release Valve – Current Too Low or Open Circuit on ECU (D420) Pin (C4)2025-12-03T16:14:26Z PACCAR MX (2004 Emissions) Fault Code: P109C Common Rail Pressure Release Valve – Current Too Low or Open Circuit on ECU (D420) Pin (C4) POSSIBLE CAUSES:No repairs are necessary. ADDITIONAL INFORMATION:Do not troubleshoot this DTC. Back to blog