To reset the readiness monitors, a drive cycle should performed.
Sometimes it requires more than one drive cycle to get all the monitors to reset.
OBD-II Drive Cycle
(Taken from Understanding OBDII: Past, Present & Future)
The OBD-II drive cycle begins with a cold start (coolant temperature below
122 degrees F and the coolant and air temperature sensors within 11 degrees
of one another).
NOTE: The ignition key must not be on prior to the cold start otherwise the
heated oxygen sensor diagnostic may not run.
As soon as the engine starts, idle the engine in drive for two and a half
minutes with the A/C and rear defrost on. OBD-II checks oxygen sensor heater
circuits, air pump and EVAP purge.
Turn the A/C and rear defrost off, and accelerate to 55 mph at half
throttle. OBD-II checks for ignition misfire, fuel trim and canister purge.
Hold at a steady state speed of 55 mph for three minutes. OBD-II monitors
EGR, air pump, O2 sensors and canister purge.
Decelerate (coast down) to 20 mph without braking or depressing the
clutch. OBD-II checks EGR and purge functions.
Accelerate back to 55 to 60 mph at half throttle. OBD-II checks misfire,
fuel trim and purge again.
Hold at a steady speed of 55 to 60 mph for five minutes. OBD-II monitors
catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors and
Decelerate (coast down) to a stop without braking. OBD-II makes a final
check of EGR and canister purge.