Instructions / Assembly

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 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.

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.

Mode $03 - Displays the type of powertrain or emission related

DTCs stored by a 5 digit code identifying the faults. There may be

more than one response message if there are more trouble codes than

will fit in the data bytes of the response message, or if there are more

than one ECU computer responding.