[loi]

I had no heat this morning when I first turn heating mode on. I touched fire box, just a cold feel on panel. I heard some low humming sound inside fire box. Where should I start looking ? I had a central Air conditioning, heating in the house.
Please help.

[loi]

Tru the hole of heater box , I see 3 red light flash, which meaned the pressure sw stucked open. What other measurement should I take before I decide to replace this Switch ? Thanks for any advise.