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This is a draft pressure switch error. 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. 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. 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.
Did you jump out the draft switch after the inducer starts up?
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