Diagnostic Overview
Mack fault code U029E-00 usually indicates lost communication with the other NOx sensor module in the SCR system. The truck may derate because it cannot verify NOx conversion. The code should be diagnosed as a communication fault before replacing SCR or DEF components.
Likely Causes
Failed NOx sensor; wiring damage; power or ground issue; CAN communication fault; corroded connector; low voltage.
What To Check First
Check sensor power and ground, inspect the harness and connector, verify CAN communication, and confirm whether other U-codes are active.