Lennox furnace problem

Hello, I am new to this forum. I found the forum while searching for my Lennox G61MPV problem. I don't want to start a new thread and I don't want to hijack OP but I have the same unit. If I need to start a new thread, let me know. My diag code is flashing 2+3 which is low pressure failed open. My unit sits horizontally.

Exhaust vent to top of roof. Intake pipe is open to attic space. There seem to be some debris in the exhaust pipe. I took a 15' flexible electrical fish rod and shove it down the exhaust flue pipe from the roof. I had a small container on the other end to catch the debris. I removed the many rubber tubes inside the unit and blow it out with compressed air and brown gunk comes flying out of tee pipe.

I have many questions and hope someone will educate me on this unit. I am not HVAC guy so any explanation will be greatly appreciated.

While searching on the internet, I see many folks with Lennox high efficiency unit show a diag code of 2+4 (High pressure switch). Mine show a 2+3 code (Low pressure switch).

Could some one educate me on what the low pressure switch regulates and what the high pressure switch regulates?

Second question, the black box trap, does it have a switch or float inside this black box?

Third question, does the black box need to have some water in it for the heat furnace to run?

Btw, AC run fine Or (atleast that what it appears to me).

TIA.