Diagnostic Trouble Code (DTC)
A code a vehicle stores when it detects a fault. Read at the shop, or increasingly reported automatically by telematics.
A diagnostic trouble code records that a control module saw something outside expected parameters. It says where to look, not what is wrong.
Telematics changed what a fleet can do with them, because codes now arrive as they occur rather than when the vehicle next visits. That makes it possible to catch a developing fault early, and it creates a new problem: a fleet receiving every code from every vehicle will drown unless it decides which ones warrant action. Choosing that filter is the actual work.
