7
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 emission 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.
2.7
OBD II Modes of Operation
Here is a basic introduction to the OBD II 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:
Mode $01
-
Identifies the Powertrain information and shows
current data available to the scan tool.
This data includes: DTCs
set, status of on-board tests and vehicle data such as engine RPM,
temperatures, ignition advance, speed, air flow rates, and closed loop
status for fuel system.
