Gas Water Heater

AO Smith water heater would not start but the pilot was lit. This one has a remote vent fan that is a few feet away from the heater in the vent pipe. While trying to check for voltage ect. I shorted out the gas pressure switch and the burner lit!! With the cable unplugged the resistance across the switch was 0 so I thought it was working. Does this pressure switch come on with the gas being on, the pilot is lit, or the burner is lit?? Can anyone explain how these things work and how to trouble shoot? The only other symptom was the vent fan would run for a few seconds randomly without the burner lighting. Thanks in Advance