Hi. Have an ultra sx80 9 yrs old. Pilot lights fine but burner doesn't always fire up. Replaced the ignitor-- no change. Everything seems to sequence properly. Bypassed all limits one at a time and checked operation of pressure switch. All seem OK. Will run for awhile and then just start blowing cold air. Must shut power down and turn off tstat. Wait several minutes, and usually will start up OK. Sometimes need to repeat the process. Out of frustration tapped on smart valve and she fired up. Im guessing that's the problem. Any input would be appreciated as its an expensive part and wanted to be sure I was looking in the right direction.

Smart valve = Dumb valve. They can be trouble. Usually the wires at the valva do not make good connections. If you order a new one make sure it is a current date code. There are still some older ones in stock and you do not want one of those unless you want more trouble in the future.