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2.6 OBD II Definitions
Powertrain Control Module (PCM)
OBDII terminology for the on-board computer that controls engine and
drivetrain.
Malfunction Indicator Light (MIL)
Malfunction Indicator Light (Check Engine) is a term used for the light on
the instrument panel. It is to alert the driver and/or the repair technician
that there is a fault with one or more of vehicle’s systems and may cause
emissions to exceed its limitation. If the MIL illuminates with a steady
light, it indicates that a fault has been detected and the vehicle should be
repaired as soon as possible. Under certain conditions, the dashboard light
will blink or flash. This indicates a severe problem and flashing is intended
to discourage vehicle operation. The vehicle on-board diagnostic system
cannot turn the MIL off until necessary repairs are completed or the
condition no longer exists.
Diagnostic Trouble Codes (DTC)
Diagnostic Trouble Codes that identify which section of the emission
control system has malfunctioned.
Enabling Criteria
Also called Enabling Conditions. They are the vehicle specific events or
conditions that must occur within the engine before the various monitors
will set or run. Some monitors require the vehicle to follow a prescribed
“drive cycle” routine as part 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 on-board diagnostics. Some form of a drive cycle needs
to be performed after DTC’s 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, refer to your
vehicle manufacturers workshop 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.
Descriptions of the modes:
Mode $01
Identifies the Powertrain information and shows current data
available to the scan tool. This data includes: DTC’s 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 on-board diagnostics parameter
IDs (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
Oxygen Sensor Test Results. This mode displays the oxygen
sensor monitor screen and the test results gathered by the
oxygen sensor.
Mode $06
Non-continuously monitored systems test results. There are
typically a minimum value, a maximum value, and a current
value for each non-continuous monitor. This data is optional,
and it is defined by a given vehicle make if it’s used
Mode $07
Request for DTCs (pending) from continuously monitored
systems after a single driving cycle has been performed
to determine if repair has fixed a problem. This is used by
service technicians to verify repair was performed properly
and after clearing diagnostic trouble codes.
Mode $08
This special control mode requests control of the on-
board system to test or component bi-directionally (where
applicable). This mode is manufacturer specific.
Mode $09
Reports vehicle information. This information includes vehicle
VIN number and calibration information stored in the vehicle
ECU’s.
Mode $10
Requests emission related diagnostic trouble codes with
permanent status. This mode is required for all emissions
related DTC’s. The presence of permanent DTC’s at an
inspection without the MIL illuminated is an indication
that a proper repair was not verified by the on-board
monitoring system.
Available 02 sensor tests
$01
Rich-to-Lean O2 sensor threshold voltage
$02
Lean-to-Rich O2 sensor threshold voltage
$03
Low sensor voltage threshold for switch time measurement
$04
High sensor voltage threshold for switch time measurement
$05
Rich-to-Lean switch time in ms
$06
Lean-to Rich switch time in ms
$07
Minimum voltage for test
$08
Maximum voltage for test
$09
Time between voltage transitions in ms
2.7 OBD II Modes of Operation
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 request, the 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
2. General information cont.
