Diagnostic Overview
U185B is a network or communication-type fault involving an aftertreatment-related module, sensor, or controller on the PACCAR MX platform. Communication faults can make the ECM lose reliable data from DEF, SCR, NOx, or aftertreatment control components and may trigger warnings or derate logic. On PACCAR MX engines, confirm the exact definition with DAVIE4, the engine serial number, emissions year, and current calibration because code meanings and troubleshooting trees can vary.
Likely Causes
Loose connector; damaged CAN wiring; low battery voltage; poor ground; failed aftertreatment module or sensor; water intrusion; harness rub-through; incorrect module programming or configuration.
What To Check First
Check battery voltage and grounds, inspect connectors and harnesses near aftertreatment components, verify CAN communication, look for related U-codes, and confirm module power, ground, and data-link integrity.