Diagnostic Overview
Mack fault code P21CA00 is generally tied to reductant/DEF or aftertreatment control performance depending on the specific Mack/Volvo engine calibration. It should be treated as an emissions-system fault that may affect SCR operation, DEF dosing, or aftertreatment monitoring. Exact definition should be confirmed in Tech Tool for the truck VIN.
Likely Causes
DEF system fault; sensor rationality issue; wiring or communication issue; dosing performance issue; contaminated DEF; calibration-specific aftertreatment fault.
What To Check First
Confirm the exact description in Tech Tool, then check DEF level and quality, related SCR codes, module communication, and DEF dosing data.