My furnace starts and stops after trying to ignite. I have changed the ignitor (it lights), the flame sensor(all burners ignite) the inducer works and then the furnace shuts down, I took out the filter to see if it was receiving enough draft and it was. Could this be the pressure switch and why?

Thanks,

The unit lights, but after a few seconds it goes out? Sure sounds like a dirty flame sensor (not to be confused with the igniter), but you say you replaced that? No need to replace it, just need to clean it with steel wool or sandcloth. If that doesn't do the trick, check and clear all your condensate drain hoses and connecting ports. Let me know what you find. If it is a pressure switch issue, the diagnostic light on the control board should be giving you an error code.