Just saw your post - I had a very similar problem this spring - furnace would turn on a few minutes, then shut down. Pulled off the covers to the blower from the outside to investigate the sail switch. Resistance measurements showed it was working properly, however, it was installed such that the "sail" was rubbing against the outside housing - installed a little shim under the switch to allow the "sail" to clear the housing, and everything is working again. Not sure why it started rubbing against the housing after 10 years of intermittent use?
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