Readiness Monitors

Definition

Readiness Monitors are automated self-diagnostic routines evaluated continuously by the vehicle's engine management computer. These internal tests check whether the vehicle's emissions control components—including the EVAP system, catalytic converter, O2 sensors, and secondary air system—are operating within environmental parameters.

Readiness monitors are categorized as either continuous (always running, such as misfire and fuel system monitors) or non-continuous (evaluated only under specific driving conditions). A non-continuous monitor requires a specific "drive cycle" (combining highway driving, stop-and-go driving, and cold-soak periods) to complete its self-test. If the battery is disconnected or trouble codes are cleared, all monitors reset to "Not Ready" or "Incomplete".

State vehicle safety and emissions inspection stations check readiness monitors through the obd2/">OBD-II port. If too many monitors are incomplete, the vehicle will fail inspection automatically, even if no active trouble codes are stored. Common faults blocking monitor completion include EVAP purge failures (P0443), thermostat slow response (P0125), or catalytic converter deterioration (P0420).