Moorio.........
You might want to include the specifics of your rig in your signature for future assistance. Identifying specific details always helps.
From what I've read from you, it appears you have two furnaces.........not all models do.
Sounds as simple as the fan for one of them is not working, compared to the rear one you mention which does work. Or .........is there that much debris stuck in the duct work from the assembly line?
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Ken and Terry
2018 Sunseeker 2430S-CD, nicely modified and carried by a 2017 Ford E450 Sport
Former Georgetown 330TS owner for 10 years with more mods than I can count, pushed by our 2017 GMC Terrain