Unit: Carrier WeatherMaker 9200 gas furnace, with an HSI ignitor
Symptom 1: Thermostat is calling for heat, as evidenced by a flame icon on thermostat face, but no action in the furnace.
Symptom 2: Ignites only AFTER I initiate Action 2 below. Then it seems to work for a while, until the problem recurs, maybe many hours later.
Diagnostic LED codes: none
Action 1: Reset the gas control switch -- no effect
Action 2: Removed cover from burner compartment. When system is calling for heat, this seems to work -- the HSI soon glows red hot, the gas ignites, and no problem -- for a while. It initially failed Friday night, worked OK most of Saturday and Sunday, but failed again Sunday night. Worked for a few hours Monday morning, but failed again after that.

Recent history: About a month ago, it was also not igniting, flashing LED error code 31 = pressure switch (and some other possible components). Not really sure what "solved" the problem, but it disappeared after playing with the unit for a while, resetting gas control switch, pressing the manual reset limit switch on the side of the burner compartment, etc. Unfortumately, not sure what actually solved the problem -- or if it solved -- because I didn't just do one thing at a time.

Questions:
1. Is temporarily taking the cover off the burner compartment -- long enough to get it to ignite -- safe?
2. Any ideas as to what the real problem is?

I have no idea what a initial combustion chamber cover is but if there are any wires around it I would be looking for a short circuit or loose wiring. Covers are usually there for a reason so removal of it is probably not a good idea.

You could be having multiple problems since it flashed a 31 code and you never found out what was causing that.

By "initial combustion chamber", I meant the burner compartment. See edited language above.

The burner compartment has 2 wires attached to what may be a temperature sensor just inside the compartment, since the cover has a decal that refers to the manual reset limit switch and says it will shut down if the temperature gets too hot.

Yes that is exactly what the control is there for it acts as a safety control. By removing that cover as you call it the control is not overheating and that will allow the unit to run. It is possible the sensor/control is out of range or defective. If that control is causing the problem that would be the first obvious part to replace.

You can test this control by possibly jumping it out of the system on a TEMPORARY basis fir test purposes only. Do not leave the furnace unattended while testing since this is a safety device. By eliminating the control on a temporary basis you will be able to tell if the unit will fire off on multiple tries. Just use the power switch on the side of the furnace to turn the furnace on and off. Test 4 or five times to be sure it is working correctly. REMOVE the jumper after the test.