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Steve you could have an issue with debris or condensation building up in the little rubber hose if it has one.Another common issue is a restriction in vent such as a spider web.You could also just have a failing board that is not sensing switch closed.
First step in troubleshooting this is going to be to get a manometer and check to see if the motor is actually drawing sufficient vacuum to close the switch.
With unt off we should not read any ohms.Switch should be open
If we get an ohms reading switch is closed. Does it go to zero when of?
Ok that will not tell us anything about the switch
I suspect board being that it is itermittent but would want to verify that vacuum before condemning.I we can verify sufficient vacuum we will know it is board not recognizing closed.
I do not have anything to do with fees. I am only sent he question and then compensated if you rate answer 3 or higher. I usually suggest to wait until we are finished to rate.Since I am going out of town I am going to opt out so another expert can take over when you are ready to continue.
It will be on a label on the pressure switch.See example below