Equus 3150 - ABS/SRS + OBD2 Diagnostic Tool - Manual (Page 26)

Onboard Diagnostics
OBD2 MONITORS
24 OBD2
Misfire Monitor - This Monitor continuously checks for engine misfires.
A misfire occurs when the air-fuel mixture in the cylinder does not
ignite. The misfire Monitor uses changes in crankshaft speed to sense an
engine misfire. When a cylinder misfires, it no longer contributes to the speed
of the engine, and engine speed decreases each time the affected cylinder(s)
misfire. The misfire Monitor is designed to sense engine speed fluctuations
and determine from which cylinder(s) the misfire is coming, as well as how
bad the misfire is. There are three types of engine misfires, Types 1, 2, and 3.
- Type 1 and Type 3 misfires are two-trip monitor faults. If a fault is sensed
on the first trip, the computer temporarily saves the fault in its memory as
a Pending Code. The MIL is not commanded on at this time. If the fault is
found again on the second trip, under similar conditions of engine speed,
load and temperature, the computer commands the MIL “On,” and the
code is saved in its long term memory.
- Type 2 misfires are the most severe type of misfire. When a Type 2
misfire is sensed on the first trip, the computer commands the MIL to
light when the misfire is sensed. If the computer determines that a
Type 2 misfire is 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 misfire is no longer present, the MIL
reverts to steady “On” condition.
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 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.
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. This Monitor is also a “Two-Trip” Monitor.