My pilot won't ignite, and I get 3 flashes from the indicator light, stating either my pressure switch is jammed or vent is blocked, how do I know which is the problem, and is this something I can repair or do I need a specialist?

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You will have to locate the pressure switch, check the vacuum hose and switch for water or other blockages, check the vacuum port in the draft inducer motot for rust or blockage, If these don't fix it you will have to pull apart the exhaust and check the flue pipe for blockages. If your furnace is 90% efficient never mind this post because there is more you should know. By the way who made your furnace and about how old is it?