How can I diagnose an intermittent furnace ignition failure?

Question

I have been researching how to diagnose this intermittent lighting issue with my furnace but the common issues do not seem to apply.

Blower turns on
Ignition coil turns cherry red
Sound of gas blower starts
No ignition, ignition coil dims, and blower remains on

I've uploaded a video as well: https://youtu.be/Y9umhFh6_gc

I am trying to figure out a strategy for determining what part I may need to order and replace.

Martin Dale Lyness · Answer

Ended up having a technician come out in fear of the intermittent failure becoming permanent in this cold winter.

Issue with this old furnace turned out to be that the gas valves were dirty blocking gas flow to some degree. The technician disassembled and cleaned the valves and went on his merry way.

I learned this is a good yearly maintenance for old furnaces like this.

How can I diagnose an intermittent furnace ignition failure?

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