Heavy Duty & OBDII Diagnostic Tool HD V3.0
6
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 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 be reset. For vehicle specific drive cycle, consult the
service 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.
Fuel Trim (FT) -- Feedback adjustments to the base fuel schedule.
Short-term fuel trim refers to dynamic or instantaneous adjustments.
Long-term fuel trim refers to much more gradual adjustments to the fuel
calibration schedule than short-term trim adjustments. These long-term
adjustments compensate for vehicle differences and gradual changes that
occur over time.
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