Purge Flow Sensor: Description and Operation

EVAP Vacuum Switch:

The EVAP vacuum switch is a normally closed switch positioned in the purge line between the canister and the EVAP purge solenoid. The control module uses the EVAP vacuum switch 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 battery positive voltage signal to the control module as a NO PURGE signal. When canister purging occurs, the switch opens, interrupting the battery positive voltage signal to the control module. A scan tool display will indicate that purge is occurring.

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

If the EVAP switch signal circuit is open when vacuum sufficient to open the EVAP vacuum switch should not be present, DTC P1442 will set.

The EVAP Vacuum Switch should be closed to ground with no vacuum present (0% EVAP Purge PWM). With EVAP Purge PWM at 25% or greater, the EVAP Vacuum Switch should open.