What are OBD readiness monitors and why are they important?

OBD readiness monitors are self-test routines that verify emissions system functionality. Key monitors include catalyst, oxygen sensor, EGR, evaporative system, misfire, and fuel system. Each monitor has enable conditions (temperature, speed, time) and runs diagnostic tests when conditions are met. Monitors report as complete when tests have run successfully. I/M (Inspection/Maintenance) testing requires monitors to be complete to pass emissions inspection. After battery disconnect or code clearing, driving under various conditions is needed to complete all monitors.