Posted for "Wally Bestgen" <whiskeyb@sbcglobal.net>:

 Hamid,
 The solenoid closes the fuel vent.  The theory is:  if the current draw is
zero the selonild has failed and the vent is open, making it possible under
certain filght conditions to allow air into the fuel system.  This detector
tells the pilot that the solenoid is not producing any resistance and must be
assumed to have failed.  Same with the pitot and prop heat.
 Wally