Evaporative Emission Monitoring

EVAPORATIVE EMISSION (EVAP) PURGE SYSTEM VACUUM SWITCH
The EVAP system uses a vacuum switch located in the EVAP purge line between the EVAP canister and the EVAP canister purge valve to detect when purge is occurring. When no purge is commanded, the switch is closed, grounding the 12 volt signal from the PCM indicating NO PURGE. When canister purge is commanded, the EVAP vacuum switch opens the ground circuit indicating PURGE.

DIAGNOSTIC FAILURE
Clogging of the canister fresh air vent could allow the purge hose between the switch and canister to trap vacuum with the purge valve closed. This would result in a diagnostic indication of a purge valve stuck open or a vacuum switch failure. Similarly, leaks or blockages in the purge hoses may result in misdiagnosis of the purge valve or vacuum switch.