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
NOTE:: Readiness will usually set quickly for most systems as long as the vehicle has no faults. But if it has a problem, the
readiness status of a particular monitor may be slow to set as the PCM tries to verify the fault. This may take several drive trips
before the PCM can determine if a temporary glitch is a real problem or not.
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:
* 1996 Chrysler vehicles may forget their readiness status when the ignition is turned off. Chrysler has a flash update for the
PCM that corrects this condition.
* 1996-1998 Mitsubishi vehicles require a very specific driving procedure (which can be found in their service literature) to run
the catalyst monitor. Because it is such a pain to run, these vehicles can be tested regardless of the catalyst monitor status.
* 1996 Nissan vehicles, and 1997 Nissan 2.0L 200SX. These vehicle may also have a "not ready" catalyst monitor because of
the driving requirements to run this monitor. The recommended drive cycle to get the catalyst monitor to run can be found in
Nissan Technical Service Bulletin #NTB98-018.
* 1996-1998 Saab vehicles are also slow to run the catalyst monitor and the EVAP monitor due to the driving requirements
programmed into their OBD II systems. Refer to Saab literature for detailed instructions on how to get these monitors to run.
* 1996 Subaru vehicles forget their readiness status when the ignition key is turned off. These is no flash reprogram for these
vehicles -- so don't turn the key off prior to an OBD II emissions test. For further info , see Subaru TSB #11-49-97R.
* 1997 Toyota Tercel and Paseo show an EVAP monitor on a scan tool but the EVAP monitor never runs! So if you are waiting
for the EVAP monitor to be ready, you will be waiting forever. No fix for this factory glitch.
* 1996 Volvo 850 Turbo cars will forget their readiness status when the ignition key is turned off. No flash update for this one
either. Refer to Volvo TSB #SB-2-23-0056.
* 1996-1998 Volvo cars (excluding the 850 Turbo) are slow to run the catalyst and EVAP monitors. Refer to Volvo TSB
#SB-2-23-0056 for drive cycle instructions on how to get these monitors to run.
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:
* Misfire monitor
* Fuel monitor
* Continuous monitor
* Oxygen sensor monitor
* EGR monitor (if used)
* Secondary Air monitor (if used)
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
NOTE: Most scan tools can access OBD II "Mode 06" diagnostics. If you go to the Mode 06 menu (which is usually found by
choosing "generic" or "global" OBD II on your scan tool main menu rather than entering the vehicle specific year, make and
model), Mode 06 data can show you if there are any pending faults that may set a code later. Mode 06 lists each component
and whether or not it is functioning within normal limits. If an item is not, it will FAIL the self-test and may set a code if it fails a
second test on a later drive cycle. The catalyst, EVAP and EGR monitors may not run if there are other DTCs present.
For more information on getting the catalyst and EVAP monitors to run, see the section on Drive Cycles. See also Mode 06