A vehicle’s OBD system has up to 12 monitors within the OBD computer. These monitors are
designed to check the running condition of all key emissions components of the engine. Three
monitors run all the time when the engine is running, the remaining monitors run intermittently
during normal driving. These monitors report to the OBD computer if the engine’s emission
performance is, or is not, in compliance with federal EPA regulations. These monitors also report
to the On-board computer as being “ready” or “not ready”. If they are “not ready,” they are not yet
in a condition to determine if the associated emissions systems are working correctly. When a
vehicle is taken to an emissions inspection station to receive the OBD I/M Check, the first thing the
testing equipment does is check these monitors. For model year vehicles 1996-2000, a vehicle with
two monitors that are “not ready” will be accepted for the OBD test and the test can continue; for
model year vehicles 2001 and newer, only one “not ready” monitor will be accepted for testing and
the test can continue. Any vehicle that has more than the allowable number of “not ready”
monitors will be rejected from testing until the monitors have been set to “ready” which may be
accomplished by performing a drive cycle specific to the vehicle.
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