Guys keep in mind that most folks asking questions on here have no idea what they are doing so keep their safety in mind when answering.
Taking the flue off to see if the furnace starts because a pressure switch isn't making is one example. Had three furnaces in the last two weeks producing between 300 and 2000 ppm of CO in the flue...how long before someone is laying on the floor dead with those levels and the flue off?
My furnace won't work...it's 20,30,40 years old. Seen it time and time again when folks put off maintenance or inspections for years and the heat exchanger is cracked or split wide open. Maybe,just maybe, telling them to put a new thermocouple in or cleaning a flame sensor to get it running isn't such a good idea.
Put a jumper across a limit,roll out or pressure switch to see if it starts. They see jumper the safety...woohoo we have heat and will get a part in a week or two. That safety is going out for a reason. They need to find the problem with a meter and not a set of jumper wires. Just recently a poster states they jumpered a pressure switch for some heat but will keep an eye on it...from the other room while watching tv.
Bottom line is they read what they want and skip a lot of the important stuff. Keep em safe even if it means no heat for a night.
OK... lecture from dad/safety czar over.