User guide

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before sending it through the rest of the exhaust. Two common cat-
alytic converters used today are: 1) Oxidizing; and, 2) Three-way.
An Oxidizing Catalytic Converter uses extra oxygen (See Second-
ary Air) to increase the rate of the chemical reaction. The Three-
Way Catalytic uses special materials (platinum, palladium, rhod-
ium, alumina, and cerium) to increase the rate of the chemical reac-
tion.
Catalyst Monitor
This monitor tests the performance of the catalytic convertor.
Before the monitor will run, certain criteria must be met: 1) the
engine must be warm; 2) the throttle must be open; 3) the fuel status
must be in a closed loop; 4) the engine RPM must be within a cer-
tain range; and, 5) the MAP must be at a specified voltage. This
does not guarantee that monitor’s test will run (i.e., an O2 sensor
DTC is stored, or a Fuel Trim to Rich/Lean is stored). The actual
test is accomplished by comparing the pre-CAT oxygen sensor’s
switching frequency, and after the post-CAT oxygen sensor’s read-
ings. If the ratio between the oxygen sensor’s switchs is outside of a
manufacturer specific threshold, the catalytic convertor is consid-
ered faulty. Catalytic converters generally don’t go bad. If the vehi-
cle’s catalytic converter is considered faulty, an effort should be
made to determine if something else is at fault. Typically, if an
engine misfires excessive heat or fuel contamination (i.e., blown
head gasket, or ring blow-by, etc), oil or coolant can bond with the
catalytic converter’s materials and cause premature failure. This is
a "Two-Trip" moniter. See also Catalytic Converter.
Closed Loop
When the monitored sensors feed back into the system the actual
and desired values, the engine computer can use the difference as
an input to reduce the error to zero. Typically when a vehicle is at
operating temperature, the fuel system is operating in a closed loop.
Continuously Monitored
A monitor that runs continuously during normal operations. This
monitor looks at a set of components that could make the engine
run out of its emissions range. The sampling rate for OBD-II, under
this definition, is no less than two samples per second.