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severe, and may cause catalytic converter damage, it commands the MIL
to “flash” once per second as soon as the misfire is sensed. When the
misre is no longer present, the MIL reverts to steady “On” condition.
The Misfire Monitor is supported by both “spark ignition” vehicles and
“compression ignition” vehicles.
2. Fuel System Monitor
This Monitor uses a Fuel System Correction program, called Fuel Trim, inside
the on-board computer. Fuel Trim is a set of positive and negative values that
represent adding or subtracting fuel from the engine. This program is used
to correct for a lean (too much air/not enough fuel) or rich (too much fuel/not
enough air) air-fuel mixture. The program is designed to add or subtract fuel,
as needed, up to a certain percent. If the correction needed is too large and
exceeds the time and percent allowed by the program, a fault is indicated by
the computer.
The Fuel System Monitor is supported by both “spark ignition” vehicles and
“compression ignition” vehicles. The Fuel System Monitor may be a “One-Trip”
or “Two-Trip” Monitor, depending on the severity of the problem.
3. Comprehensive Components Monitor (CCM)
This Monitor continuously checks all inputs and outputs from sensors,
actuators, switches and other devices that provide a signal to the computer.
The Monitor checks for shorts, opens, out of range value, functionality and
“rationality* (
See Note
).”
Raonality: Each input signal is compared against all other inputs and against
informaon in the computer’s memory to see if it makes sense under the current
operang condions.
Example: The signal from the throttle position sensor indicates the vehicle is
in a wide-open throttle condition, but the vehicle is really at idle, and the idle
condion is conrmed by the signals from all other sensors. Based on the input
data, the computer determines that the signal from the throle posion sensor
is not raonal (does not make sense when compared to the other inputs). In this
case, the signal would fail the raonality test.
The CCM is supported by both “spark ignition” vehicles and “compression
ignition” vehicles. The CCM may be either a “One-Trip” or a “Two-Trip”
Monitor, depending on the component.
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