Posted by Alex (Im) E. on 01 February 2013 12:17 AM
MONITOR READINESS STATUS
Readiness flags are indicators on a scan tool or code reader that tell you when an OBD II system monitor has run or completed its test. A scan tool may say "ready" or "Complete" next to a listed monitor if it has run, or "not ready" or "incomplete" if it has not run. Others display a little flag or dot to indicate when a monitor has run.
If all the OBD II monitors have run, the OBD II system is ready. Ready for what? Ready for an OBD II emissions test or an I/M 240 or ASM loaded mode emissions test.
The emission test rules require that OBD II run most or all of its self checks BEFORE a vehicle is tested.
Why? So people don't use a scan tool or code reader to clear codes just before the test so they can pass the test.
If all the monitors have run and are ready, it guarantees an honest and accurate emissions test. And if all the monitors have run and are ready -- and OBD II has NOT found any faults, the MIL lamp is OFF and there are NO CODES in memory -- it means the vehicle should be in emissions compliance and pass any type of test it is given (OBD II, I/M 240 or ASM).
For 1996 through 2000 model year vehicles, the EPA emissions test rules allow up to two (2) readiness monitors NOT to be set when the vehicle is tested. Why? Because many vehicles require quite a bit of driving before all the monitors will run and be ready.
A short trip to an emissions test center may not involve enough driving or the right driving conditions to set the catalyst monitor and/or EVAP monitor (which are the two hardest monitors to complete because they require specific operating conditions before they will run).
On 2001 and up model year vehicles, all monitors must be ready before the vehicle can be given an OBD II test (rules may vary somewhat from one state to another). Exceptions may also be made for certain vehicles that have known readiness issues.
Some vehicles may forget their readiness status if the ignition key is turned off just prior to a test.
On others, the catalyst or EVAP monitor may only run under certain conditions -- which may be difficult to recreate prior to an emissions test.
The following are some of the vehicles with such issues:
READINESS AND BLOCKED CODES
In some instances, a fault that sets an OBD II code may prevent an OBD II monitor from running and detecting any additional faults.
In other words, if one monitor runs and finds a fault, it may block or prevent subsequent monitors from running and completing the entire list of OBD II self-checks.
Consequently, you may find a code, diagnose and fix a problem only to see the MIL light come back on when the vehicle is driven.
What's happening? The OBD II system is trying to complete its job. Now that the first problem has been fixed, it can keep running all its monitors until the system check is complete. And if it finds any additional faults, it will sets additional codes and turn on the MIL light.
To prevent "blocked codes" from happening, the following monitors should all be run:
Also, if a repair has involved the catalytic converter or a component in the evaporative emissions system, the vehicle should be driven long enough to ensure the catalyst monitor and EVAP monitors have both run and are ready (and hopefully no fault found).
For more information on getting the catalyst and EVAP monitors to run, see the section on Drive Cycles.