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3001
User’s Manual
10
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).”
*Note: Rationality: Each input signal is compared against all other inputs and against
information in the computer’s memory to see if it makes sense under the current
operating conditions.
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 condition is
confirmed by the signals from all other sensors. Based on the input data, the computer
determines that the signal from the throttle position sensor is not rational (does not
make sense when compared to the other inputs). In this case, the signal would fail the
rationality 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.
2.6.2 Non-Continuous Monitors
The other twelve Monitors are “non-continuous” Monitors. “Non-continuous”
Monitors perform and complete their testing once per trip. The “non-continuous”
Monitors are:
1. O2 Sensor Monitor
The Oxygen Sensor monitors how much oxygen is in the vehicle’s exhaust. It
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