6
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.
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.
Mode $04
–
used to clear DTCs and Freeze Frame data
. This clears all
diagnostic trouble codes that may be set including freeze frame data and
readiness monitors.
Mode $05
–
displays oxygen sensor test results.
This mode displays the
oxygen sensor monitor screen and the test results gathered about the
oxygen sensor.
There are ten numbers available for diagnostics:
1. $01 Rich-to-Lean O2 sensor threshold voltage
2. $02 Lean-to-Rich O2 sensor threshold voltage
3. $03 Low sensor voltage threshold for switch time measurement
