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14
DN606 Automotive Diagnostic Tool User's Manual
The computer checks the efciency of the catalytic converter by monitoring the
oxygen sensors used by the system. One sensor is located before (upstream
of) the converter; the other is located after (downstream of) the converter. If the
catalytic converter loses its ability to store oxygen, the downstream sensor signal
voltage becomes almost the same as the upstream sensor signal. In this case,
the monitor fails the test.
The Catalyst Monitor is supported by “spark ignition” vehicles only. The Catalyst
Monitor is a “Two-Trip” Monitor. If a fault is found on the rst trip, the computer
temporarily saves the fault in its memory as a Pending Code. The computer does
not command the MIL on at this time. If the fault is sensed again on the second
trip, the computer commands the MIL “On” and saves the code in its long-term
memory.
4. Heated Catalyst Monitor
Operation of the “heated” catalytic converter is similar to the catalytic converter.
The main difference is that a heater is added to bring the catalytic converter to
its operating temperature more quickly. This helps reduce emissions by reducing
the converter’s down time when the engine is cold. The Heated Catalyst Monitor
performs the same diagnostic tests as the catalyst Monitor, and also tests the
catalytic converter’s heater for proper operation.
The Heated Catalyst Monitor is supported by “spark ignition” vehicles only. This
Monitor is also a “Two-Trip” Monitor.
5. EGR (Exhaust Gas Recirculation) System Monitor
The Exhaust Gas Recirculation (EGR) system helps reduce the formation
of Oxides of Nitrogen during combustion. Temperatures above 2500°F
cause nitrogen and oxygen to combine and form Oxides of Nitrogen in the
combustion chamber. To reduce the formation of Oxides of Nitrogen, combustion
temperatures must be kept below 2500°F. The EGR system recirculates small
amounts of exhaust gas back into the intake manifold, where it is mixed with the
incoming air/fuel mixture. This reduces combustion temperatures by up to 500°F.
The computer determines when, for how long, and how much exhaust gas is
recirculated back to the intake manifold. The EGR Monitor performs EGR system
function tests at preset times during vehicle operation.
The EGR Monitor is supported by both “spark ignition” vehicles and “compression
ignition” vehicles. The EGR Monitor is a “Two-Trip” Monitor. If a fault is found on
the rst trip, the computer temporarily saves the fault in its memory as a Pending
Code. The computer does not command the MIL on at this time. If the fault is
sensed again on the second trip, the computer commands the MIL “On,” and
saves the code in its long-term memory.
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