Evaporative Emission Monitoring

EVAP System:

The EVAP system uses a switch located in the purge line between the canister and the purge valve in order to detect when the purge is occurring. This switch senses the flow from the engine through the purge valve When no purge is present, the switch is closed, applying a 12 volt signal to the control module as a NO PURGE signal. When the canister purging occurs, the switch opens, interrupting OFF the 12 volt signal to the control module. A scan tool display will indicate that the purge is occurring.

Clogging of the canister fresh air vent could allow the purge hose between the switch and the canister to trap the 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 the vacuum switch.

When servicing a purge valve diagnostic trouble code, check the canister fresh air vent, the vacuum switch and the integrity of all of the purge hoses prior to servicing the valve.