Equus 3110 - CanOBD2 Diagnostic Tool - Manual (Page 27)

CAN OBD2 25
E
Onboard Diagnostics
OBD 2 MONITORS
Misfire Monitor - This Monitor continuously checks for engine
misfires. A misfire occurs when the air-fuel mixture in the cylin-
der 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 mem-
ory.
- 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 dam-
age, 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 (car-
bon 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 mon-
itoring the oxygen sensors used by the system. One sensor is located
before (upstream of) the converter; the other is located after (down-
stream 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 com-
mands the MIL “On” and saves the code in its long-term memory.