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Figure 2-2
2.4 OBD II Readiness Monitors
An important part of a vehicle’s OBD II system is the Readiness Monitors, which are indicators used to find out if all of the emissions components
have been evaluated by the OBD II system. They are running periodic tests on specific systems and components to ensure that they are
performing within allowable limits.
Currently, there are eleven OBD II Readiness Monitors (or I/M Monitors) defined by the U.S. Environmental Protection Agency (EPA). Not all
monitors are supported in every vehicles and the exact number of monitors in any vehicle depends on the motor vehicle manufacturer’s emissions
control strategy.
Continuous Monitors — Some of the vehicle components or systems are continuously tested by the vehicle’s OBD II system, while others are
tested only under specific vehicle operating conditions. The continuously monitored components listed below are always ready:
1. Misfire
2. Fuel System
3. Comprehensive Components (CCM)
Once the vehicle is running, the OBD II system is continuously checking the above components, monitoring key engine sensors, watching for
engine misfire, and monitoring fuel demands.
Non-Continuous Monitors — Unlike the continuous monitors, many emissions and engine system components require the vehicle to be operated
under specific conditions before the monitor is ready. These monitors are termed non-continuous monitors and are listed below:
1) EGR System
2) O2 Sensors
3) Catalyst
4) Evaporative System
5) O2 Sensor Heater
6) Secondary air Injection
7) Heated Catalyst
8) A/C system
2.5 OBD II Monitor Readiness Status
OBD II systems must indicate whether or not the vehicle’s PCM’s monitor system has completed testing on each component. Components that
have been tested will be reported as “Ready”, or “Complete”, meaning they have been tested by the OBD II system. The purpose of recording
readiness status is to allow inspectors to determine if the vehicle’s OBD II system has tested all the components and/or systems.
The Powertrain Control Module (PCM) sets a monitor to “Ready” or “Complete” after an appropriate drive cycle has been performed. The
drive cycle that enables a monitor and sets readiness codes to “Ready” varies for each individual monitor. Once a monitor is set as “Ready”
or “Complete”, it will remain in this state. A number of factors, including erasing of Diagnostic Trouble Codes (DTCs) with a code reader or a
disconnected battery, can result in Readiness Monitors being set to “Not Ready”. Since the three continuous monitors are constantly evaluating,
they will be reported as “Ready” all of the time. If testing of a particular supported non-continuous monitor has not been completed, the monitor
status will be reported as “Not Complete” or “Not Ready.”
In order for the OBD monitor system to become ready, the vehicle should be driven under a variety of normal operating conditions. These
operating conditions may include a mix of highway driving and stop and go, city type driving, and at least one overnight-off period. For specific
information on getting your vehicle’s OBD monitor system ready, please consult your vehicle owner’s manual.
