I called in a technician when one of my two furnaces stopped heating consistently - it would sometimes start then shut off. They replaced the control board and recommended a few other parts to be replaced (inducer, pressure switch). They also strongly recommended I replace my other furnace, which was working, with a newer, more efficient model. I did that but the installation was poor causing a vibration throughout my first floor. Rather than rebuild the plenum they first cut into the floor joists to increase clearance (didn't work) then smashed it down which cured the vibration but bent the intake bad enough that the filter no longer fits.
A few months later, the first furnace started to fail again and I decided to go online to research the solution myself because it was late in the season and I didn't want to deal with another contractor. It turned out to be the ignitor - duh!
By the way, the markup on the furnace parts was about 200% over what I could find on the Internet.
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