03 December, 2025PACCAR MX (2004 Emissions) Fault Code: P151D Aftertreatment System – DEF Level Very Low PACCAR MX (2004 Emissions) Fault Code: P151D Aftertreatment System – DEF Level Very Low SET CONDITIONS:The DEF level is less than 15% of the DEF tank volume. DIAGNOSTIC CONDITION:This diagnostic runs continuously when the ignition is on. RESET CONDITION:This DTC becomes inactive after the ignition is keyed on and the volume in the DEF tank is above the set condition value. POSSIBLE CAUSES:Empty DEF tank ADDITIONAL INFORMATION:This DTC activates an engine torque derate of 25% and P1518 becomes active. Share on Facebook Opens in a new window. Tweet on Twitter Opens in a new window. Pin on Pinterest Opens in a new window. Back to blog