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OBD II Terminology
Powertrain Control Module (PCM)
- OBDII terminology for the on-board
computer that controls engine and drive train.
Malfunction Indicator Light (MIL)
- Malfunction Indicator Light (Service Engine
Soon, Check Engine) is a term used for the light on the instrument panel. It is to
alert the driver and/or the repair technician that there is a problem with one or
more of vehicle’s systems and may cause emissions to exceed federal standards.
If the MIL illuminates with a steady light, it indicates that a problem has been
detected and the vehicle should be serviced as soon as possible. Under certain
conditions, the dashboard light will blink or flash. This indicates a severe problem
and flashing is intended to discourage vehicle operation. The vehicle onboard
diagnostic system can not turn the MIL off until the necessary repairs are
completed or the condition no longer exists.
DTC - Diagnostic Trouble Codes (DTC) that identifies which section of the
emission control system has malfunctioned.
Enabling Criteria - Also termed Enabling Conditions. They are the vehicle-specific
events or conditions that must occur within the engine before the various monitors
will set, or run. Some monitors require the vehicle to follow a prescribed “drive
cycle” routine as part of the enabling criteria. Drive cycles vary among vehicles
and for each monitor in any particular vehicle.
OBDII Drive Cycle-- A specific mode of vehicle operation that provides conditions
required to set all the readiness monitors applicable to the vehicle to the “Ready”
condition. The purpose ofcompleting an OBD II drive cycle is to force the vehicle
to run its onboard diagnostics. Some form of a drive cycle needs to be performed
after DTCs have been erased from the PCM’s memory or after the battery has
been disconnected. Running through a vehicle’s complete drive cycle will “set” the
readiness monitors so that future faults can be detected. Drive cycles vary
depending on the vehicle and the monitor that needs to be reset. For vehicle
specific drive cycle, consult the vehicle’s Owner’s Manual.
Freeze Frame Data
- When an emissions related fault occurs, the OBD II system
not only sets a code but also records a snapshot of the vehicle operating
parameters to help in identifying the problem. This set of values is referred to as
Freeze Frame Data and may include important engine parameters such as engine
RPM, vehicle speed, air flow, engine load, fuel pressure, fuel trim value, engine
coolant temperature, ignition timing advance, or closed loop status
