Amana Gas Furnace. Furnace blower appears to work, but no heat. 3 Blinking red lights. I googled this, someone said 3 lights can mean: 1. The exterior vent pipe is blocked. 2. The pressure switch is bad or wired wrong 3. The ventor is clogged or jammed. Attempted some cleaning with shop vac to see if there was an obstruction. When shop vac was stuck into exhaust vent pipe, the furnace kicks on. As soon as I take it out, it shuts off. Not sure what the problem is, but I would like it resolved today if I can possibly do it. If not, I'll call a pro, but since I'll be calling out of the phone book, I want to minimize the chance of being ripped off by at least seeming knowledgeable. Thanks.