CLAS AC 5247 Mode D'emploi page 30

AC 5247
READER DEFAULT CODE OBDII
6. OBDII Definitions
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
necessary repairs are completed or the condition no longer exists.
DTC -- Diagnostic Trouble Codes (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.
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 of completing an OBDII 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 OBDII 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
7. OBDII Modes of Operation
Here is a basic introduction to the OBDII communication protocol. Mode byte: The first byte
in the stream is the mode number. There are 9 modes for diagnostic requests, so this first
byte is from 1 to 9. The first byte in the response data bytes is this same number plus 64.
For example, a mode 1 request would have the first data byte = 1, and the response would
have the first data byte = 65.
Here is a brief description of the modes: