100K BTU ng,Honeywell 1982 furnace.
New filters, good intake(I think). Has worked fine until last 2.5 weeks.

Pilot light is fine, main gas ignites and will stay lit for about 20 seconds. The honeywell hi/lo limit switch clock turns to main fan, sometimes it will reach the fan on temp, but rarely.

I have tried to start/restart to get temp up to main fan temp to no avail. Flame out still within 20 seconds. There is a valve shutting it down as you can notice the slow fading. Sometimes it will kick back on for a few more seconds, but then shut down again.

I assume at this time, the hi/lo limit switch is failing.
Tks

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