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12
DN606 Automotive Diagnostic Tool User's Manual
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.
2.6.2 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
generates a varying voltage of up to one volt, based on how much oxygen is in
the exhaust gas, and sends the signal to the computer. The computer uses this
signal to make corrections to the air/fuel mixture. If the exhaust gas has a large
amount of oxygen (a lean air/fuel mixture), the oxygen sensor generates a “low”
voltage signal. If the exhaust gas has very little oxygen (a rich mixture condition),
the oxygen sensor generates a “high” voltage signal. A 450mV signal indicates
the most efcient, and least polluting, air/fuel ratio of 14.7 parts of air to one part
of fuel.
The oxygen sensor must reach a temperature of at least 600-650°F, and the
engine must reach normal operating temperature, for the computer to enter into
closed-loop operation.
The oxygen sensor only functions when the computer is in closed-loop. A
properly operating oxygen sensor reacts quickly to any change in oxygen content
in the exhaust stream. A faulty oxygen sensor reacts slowly, or its voltage signal
is weak or missing.
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