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2.6 OBD II Definitions
Powertrain Control Module (PCM) — OBD II 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 cannot 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. Please refer to the vehicle’s factory service manual for specific
enabling procedures.
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 service 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.
Fuel Trim (FT) — Feedback adjustments to the base fuel schedule. Short-term fuel trim refers to dynamic or instantaneous adjustments. Long-
term fuel trim refers to much more gradual adjustments to the fuel calibration schedule than short-term trim adjustments. These long-term
adjustments compensate for vehicle differences and gradual changes that occur over time.
