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15
DN606 Automotive Diagnostic Tool User's Manual
6. EVAP System Monitor
OBD II vehicles are equipped with a fuel Evaporative system (EVAP) that helps
prevent fuel vapors from evaporating into the air. The EVAP system carries
fumes from the fuel tank to the engine where they are burned during combustion.
The EVAP system may consist of a charcoal canister, fuel tank cap, purge
solenoid, vent solenoid, ow monitor, leak detector and connecting tubes, lines
and hoses.
Fumes are carried from the fuel tank to the charcoal canister by hoses or tubes.
The fumes are stored in the charcoal canister. The computer controls the ow of
fuel vapors from the charcoal canister to the engine via a purge solenoid. The
computer energizes or deenergizes the purge solenoid (depending on solenoid
design). The purge solenoid opens a valve to allow 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 first 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 first trip, the computer temporarily saves this fault in its memory as a
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