Full Product Manual

6
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.
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.
Mode $02 – displays Freeze Frame data. Same data as in mode 1, but it
was captured and stored when a malfunction occurred and a DTC was set.
Some of the PIDs for mode one are not implemented in this mode.