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2.6 OBD II Definitions
Powertrain Control Module (PCM) -- OBD II terminology for the on-board
computer that controls the engine and the 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 Code (DTC) that identify 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.
OBD II 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 of completing 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
sets a code and records a snapshot of the vehicle operating parameters to help
identify 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.
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