Furnace fires up but no fan/heat

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I have a gas furnace (forced hot air) that was working fine a week ago. This week I noticed it wasn't blowing any hot air, in fact no air at all when the heat is on. I can hear the furnace fire up and run when I switch the thermostat to "heat on" but no warm air comes out. I thought the fan was not working - if the thermostat fan switch is moved to "on" from "auto", still no warm air. To be sure I turned the furnace off via the HVAC switch and turned the fan on, to my surprise the fan was working. I changed the batteries on the programmable Honeywell thermostat, then finally replaced it with a new Honeywell programmable. Same symptoms, furnace on but no warm air. Fan turns on when heat is in off position. I am hoping this set of symptoms will identify a failing part in the furnace. The unit is a Carrier Crusader, gas from the 90's, heat only. I cannot find a model number markings. Any help would be appreciated!
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Licensed plumbing contractor
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San Francisco Bay Area
You have established that the fan is good.
The heating sequence is that flame rod needs to detect flame on fire-up. That is a pick and hold function.
You need to watch the fire-up sequence.
Definitely pull the flame rod assy and use emery cloth to thoroughly clean it.
Hey, wait a minute.

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