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Hello my name is XXXXX XXXXX will be my pleasure to assist you. This is a pressure switch fault. Locate the pressure switch in the furnace disconnect the rubber tube connecting the inducer housing and the pressure switch check the tube for any obstructions and check the nipples for any type of obstructions. The one on the inducer housing usually gets soot and junk in it. Use a small allen wrench to poke through the nipple then use a shop vac to suck out the nipple. Also check the termination outside for any obstructions like bees, birds, Squirrels as this will also cause the pressure switch to fault.
Occassionally the furnace turns on by itself which it just did so I am waiting for it to finish and then will check what you suggested. Where about do I find the pressure switch fault
You should turn the furnace off the pressure switch is in the top of the furnace near where the vent pipes attach.
I tried this and it still does not work. Any other suggestions
Okay use an unbent paper clip and disconnect the wires to the pressure switch with the power off. Once the you turn the power back on use the unbent paperclip to jump the wies of the pressure switch if the furnace starts and runs normal then you may have a bad pressure switch or the termination of the vent pipes may be blocked check outside for any type of blockage of the vent pipes.
Ok - I will check that shortly
Once I tried that - it does work. No block outside either so I guess a bad switch. Where can I get another one
You can order them online you will need to know the i.w.c rating on the side of the switch. to order the right one.