Today our Trane XE80 (Model TUD060C936H3, with a White Rogers Model 50A50-473 control) did not ignite twice (igniter works, but I don't think it turns on, or natgas does not turn on even though blower is on) -- all other times everything works). Each time, I manually turned the furnace off (at the furnace) and the unit reignited when turned back on. (At least) once, the LED on the control unit flashed twice (which means "system lockout (no flame) check line polarity" per comments on the control unit). Thought it might be the flame sensor (from previous posters here), but when I downloaded the troubleshooting guide from the White Rogers website (Model 50A55, which it replaces), it says that 2 flashes means the pressure switch is stuck closed (one flash on the 50A55 is system lockout). But most of the time, the furnace cycles fine. Don't want us to be gone sometime and the furnace doesn't kick in. Any thoughts? Since it's Christmastime, don't know whether to try and troubleshoot this myself or call in "an expert". I've tried looking for troubleshooting info for the XE80 at Trane's website and haven't found anything yet.