On Wednesday, February 3, 2021 at 11:01:12 AM UTC-5, Jeff Gramza wrote:
Furnace is around 20 years old. Sometimes the main burners light and sometimes they don't. When they don't, I go downstairs, turn the blower motor off and flip the black switch to 'off' on the little beige box (smart valve?). Wait 30 seconds, reverse the process and the main burners come on. They may come on the next few times but when it starts to get cold in the house, I go downstairs and see the main burners haven't fired up. Doesn't appear the plastic hoses are clogged. Any suggestions where to start looking?

Are you sure they are not lighting or are they lighting, then going out? If it;s
the latter, could be a bad flame sensor. If it won't light at all, what is the lighting
method? Pilot light? Hot surface igniter? Is it high efficiency with a blower for
combustion? Could be a pressure switch, blocked vent to the outside.