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The draft pressure switch is on top of the tank and is similar to the ones in the attached pic. The question is do you have a bad switch or a weak fan or a blocked exhaust vent. You can check the vent for blockages and also the vac tubing connected to the switch. Be sure the tubing is not kinked, deteriorated or doesn't have water in it. You can also poke a paper clip into the port where the tubing connects but not into the switch connection. If all that checks out. Turn the power off to the unit wait a couple minutes then turn it back on. After the fan starts remove one of the leads from the pressure switch and connect it to the other lead. If the sequence continues then you may have a faulty pressure switch or possibly a weak fan.