Users Manual Part 2

36
MDMAXLITEA
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.
3. Catalyst Monitor
The catalytic converter is a device that is installed
downstream of the exhaust manifold. It helps to oxidize
(burn) the unburned fuel (hydrocarbons) and partially burned
fuel (carbon monoxide) left over from the combustion
process. To accomplish this, heat and catalyst materials
inside the converter react with the exhaust gases to burn
the remaining fuel. Some materials inside the catalytic
converter also have the ability to store oxygen, and release it
as needed to oxidize hydrocarbons and carbon monoxide. In
the process, it reduces vehicle emissions by converting the
polluting gases into carbon dioxide and water.
The computer checks the efficiency 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 first 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