My unit is an XE90 model tux080c942c3, serial z2833rt7g. I have had annual issues with this baby, but this year, it is not cooperating. When switched on, the exhaust fan starts up and the ignitor (new) starts to heat (I know this cause I burnt my finger on it), but before it can get to glowing, the exhaust fan kicks off and the unit clicks into the circulator fan clearing out the system. I am getting nine blinks on the unit (check ignitor). I ran a snake up the exhaust pipe and checked the roof vent as well as opened the pipe in the attic. It appears clear. I also gave the flame sensor a quick sanding to clean it off. Any thoughts? Could the exhaust fan pressure sensor be bad?