G6RC 40k BTU single pipe (exhaust) 90+ Natural gas furnace in light duty service for some 10 years. The unit has a small combustion chamber inside the cabinet with a spyglass. Standard issue hot surface ignitor and a rectifier flame sensor.
find documentation here: transtaracsupply.com/pdf/g6rc_service.pdf
Firing sequence is disrupted after proof of flame. (Gas valve opens - all jets fire, then... pffft)
That is unless the combustion chamber cover is off. Then fires up normal - put the chamber cover back on and everything runs great within operating parameters. No flames shooting back the wrong way (rollout switch) Just runs like a champ.
I'm out of ideas. Everything checks. There are no blockages (squirrels in the vent - gunk in the condensate line) The heat exchanger looks great, etc. etc.
So, unless I've got MY wires crossed (in my head) all the switches , sensors are normally open - right?
Can I jumper around all four sensors at the same time and at least see if something is being finicky at just the wrong time?
To me though it comes down to understanding why the bloody thing will fire when the combustion chamber cover is completely off. (not just ajar, slid half way off) Completely off (with the wires still attached to the rollout switch of course) I'm open to suggestions.

I'm going to answer my own question... partially. Checked micro-amps from "flame rod". Chamber closed - Zip, zilch, nada.
Chamber open - 2.5 micro-amps. Apparently a little weak - but enough to rectify the current.
Will thoroughly clean everything and expect problem will be solved.

Something is blocking the intake air, or you have a pluged up heat chamber