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engine vacuum to draw the fuel vapors from the canister into the engine where
the vapors are burned. The EVAP Monitor checks for proper fuel vapor ow to
the engine, and pressurizes the system to test for leaks. The computer runs
this Monitor once per trip.
The EVAP Monitor is supported by “spark ignition” vehicles only. The EVAP
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 PCM commands the MIL “On,” and saves the code in its long-
term memory.
7. Secondary Air System Monitor
When a cold engine is rst started, it runs in open-loop mode. During open-
loop operation, the engine usually runs rich. A vehicle running rich wastes
fuel and creates increased emissions, such as carbon monoxide and some
hydrocarbons. A Secondary Air System injects air into the exhaust stream to
aid catalytic converter operation:
• It supplies the catalytic converter with the oxygen it needs to oxidize the
carbon monoxide and hydrocarbons left over from the combustion process
during engine warmup.
• The extra oxygen injected into the exhaust stream also helps the catalytic
converter reach operating temperature more quickly during warm-up
periods. The catalytic converter must heat to operating temperature to work
properly.
The Secondary Air System Monitor checks for component integrity and system
operation, and tests for faults in the system. The computer runs this Monitor
once per trip.
The Secondary Air System Monitor is a “Two-Trip” monitor. If a fault is found
on the rst trip, the computer temporarily saves this 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.
2.6.3 OBD II Reference Table
The table below lists current OBD II Monitors, and indicates the following for
each Monitor:
A. Monitor Type (how often does the Monitor run; Continuous or Once per
trip).
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