The majority of no heat calls can attributed to one or more of five things. Poor installation, customer neglect (poor maintenance), environmental causes, equipment age, or manufacturers defects. For the most part, poor installation and maintenance are the biggest causes of problems with a furnace, but equipment age will be a close third. Plugged filters cause the most problems, but by and large, one of the most common trouble codes on a modern, computer equipped furnace will be a "pressure switch error" or other problem associated with this switch or switches (some furnaces have up to four of these).
Again, I am not responsible for the quality of your work nor can I know what else is wrong with your equipment. This is a safety device that must not be tampered with in any way nor replaced with in such a way as to defeat the purpose, or people could die or property damage could result. When in doubt, call a professional. Use this at your own risk.
All a pressure switch is a diaphragm that acts to allow something else to work, usually the ignitor or gas valve after the vent motor comes on. There's a hose that leads to a spud on the vent motor or heat exchanger and two or more terminals with wires attached. If there's any issue with the venting or drain this switch will not close and the ignitor will not come or the gas valve will not open. A clogged drain trap or drain will cause this on a condensing furnace (90% efficient or higher gas furnace) as will a blocked spud, plugged or leaking heat exchanger, blocked or leaking vent, or even a computer or thermostat will cause this error.
You will need a few tools to do this correctly and some are expensive. A manometer (to test for pressure in inches of water column), a multimeter (to test for continuity and voltage), a carbon monoxide detector (Not the kind you hang on your wall), a battery drill with 5/16 and 1/4 inch nut runners, jumpers and possibly Allen wrenches. Here's a rough procedure for testing this switch, but the most import part is NOT to condemn the switch unless you are sure it is bad or there will be a callback. Time and again, I've seen homeowners change these out and this wasn't the problem. Then they call me.
Check the vent and intake for blockage, the drain and trap, and the vent motor for operation BEFORE you even investigate the pressure switch. If the collector plate and vent motor are full of water, the pressure switch won't operate. These obvious problems be be the solution after all.
If this doesn't solve the problem, then hook up a tee with your manometer, in line with the pressure switch. If the pressure exceeds the specs on a positive switch or below on a negative switch, then you need to check to make sure you aren't losing your call for heat across the switch and that the electrical contacts are intact. If you're losing your call for heat, the thermostat may be bad or the board may be going out (this is very common on some Goodman down flow furnaces). The switch could also be calling it a day (not very common at all).
If these are all good and the problem is intermittent, then you need to break out the big guns. Turn the furnace on and break out the carbon monoxide detector. Air out the house if the homeowner is a smoker and then turn the detector on outside. Bring it in and stick the probe into the warm air registers. If the ppm is over 0 (zero) the heat exchanger is suspect and could be failing. Even if it looks good after pulling it (depending on the age of the furnace) doesn't mean it's still good. Filling the heat exchanger with water will prove this out. I did this on a job and sure enough, water gushed from the seams. The heat exchanger was the culprit.
I don't want to say this is a comprehensive or even authoritative way to test or prove out a pressure switch. Professionals will probably laugh this article off the net. What I do want to show is that troubleshooting is a means to an end and must be followed to the end to solve the problem. Throwing parts at a seemingly obvious problem will just cost more in the long run if the part doesn't fix it. In the case of replacing a pressure switch you have less than a 20% chance of fixing it. If you use the wrong part, it may cost you your life. Please, remember that troubleshooting is something that takes lots of training and practice. It isn't something that anyone should do willy nilly, but with a fair amount of knowledge and understanding on how things work. I hope this article gives the layperson some insight into the effort and knowledge that it takes to fix something and hopefully gripe a little less when the invoice is due. Maranatha!