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This indicates an open draft pressure switch. . 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 of that checks out the next thing we would do is put a manometer on the exhaust to check the actual draft pressure but you aren't likely to have a manometer and they are expensive so replacing the switch is one other option since it's less money than a manometer (or a new draft fan) and it is a likely culprit. A picture of a typical draft switch is attached