HOW DOES THE EGR VACUUM REGULATOR (EVR) SOLENOID WORK?

I HAVE A 1996 FORD RANGER V6 4.0 AND I KEEP GETTING CODE PO402 I HAVE REPLACED EVERYTHING. THE EVR SOLENOID WHEN CHECKED FOR POWER I FOUND IT IS ALWAYS HOT. WHAT TELLS THE COMPUTER TO ENERGIZE THE SOLENOID, AND HOW DOES THE COMPUTER DO THAT, DOES IT SUPPY POWER OR GROUND , ALSO COULD THE THE EGR TUBE WHERE THE HOSES FROM THE DPFE SENSOR GO SOME HOW CAUSE THIS CODE? DOES THE TEMP SENSOR HAVE ANYTHING TO DO WITH EGR FLOW?