I ran the diagnostic and it returns error code 11 which is useless. Right after power is applied to unit. The red diagnostic light comes on. According to the manual that is 24 VAC. The thermostat has a display but if I call for heat the display blanks and a red light is displayed. On the heater unit itself, I short the test/run to the 24VAC temporarily and get 1 short and 1 long 4 times. Then the blower turns on, ignitor glows then there are a couple of clicks and then everything stops and the red diagnostic light comes on again solid.

What brand and Model thermostat and furnace do you have?