Autel AutoLink AL539B [9/92] Obd ii modes of operation

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 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 10 modes for diagnostic requests. 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: DTC 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.