Problems with Duomatic Olsen Ultramax HCS-90

I've been having intermittent issues with my approximately 15+ year old furnace. The thermostat is calling for heat and the fan is on, however the furnace won't light up. It works fine if you open the furnace and switch to manual. I've checked the thermostat and it works fine. I've read where these furnaces have a lot of different problems, but I'm not sure what the issue is with this.
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