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EVAP canister purge regulator valve, checking

The EVAP system (evaporative emissions control system) filters vapors from the fuel tank through an activated carbon filter, so that fuel fumes are not let in to the atmosphere. The EVAP canister purge regulator valve lets these vapors go through the filter and into the engine's intake manifold under certain conditions. During these conditions, you may hear the valve clicking rapidly -- this is perfectly normal and nothing to worry about.

First, perform the easy electrical check. Disconnect the harness connector for the valve and measure the resistance. You should get a value of 20-28 ohms. If you do not, the valve is faulty and must be replaced.
EVAP resistance

If that value is OK, next check the battery positive voltage (B+) supply to the valve. Disconnect the harness connector on the valve, and connect your favorite voltage meter to terminal 1 and a handy ground (GND), such as the engine block.
2 pin connector

If you do not get 12 volts, check fuse 28, which should be under a red cover in the main fuse panel. If that is OK, disconnect the battery and check the resistance between pin 1 of the EVAP connector and fuse 28. There should be no more than 0.5 ohms between them.

If battery supply was OK, check the wiring between the ECU and the valve. Pin 2 of the valve's connector should be connected to ECU connector D terminal 1. If there is no connection, or if the connection is shorted to ground or B+, fix the wiring. There should be no more than 1 ohm resistance between the valve and the ECU.
ECU connectors

If all of the above checks are OK, you might need to replace the ECU.