The furnace is about 5-6 years old, and advertised as "ultra-efficient". Gas furnace is inside, Carrier Tech 2000SS A/C is outside. Heat won't come on, despite my reset efforts:
-turn thermostat to lowest setting
-turn off all electrical power to furnace
-remove access panel from furnace
-flip gas control switch to "off", wait 5 minutes, flip back to "on"
-replace access panel
-turn electrical power on
-set thermostat to desired setting

The above instructions are visible after removing the access panel, but haven't solved the problem. The A/C did come on when thermostat was lowered. Also the blower will blow when set to "on" position, but still no heat. Roommate replaced thermostat -- still no heat. The unit comes without a pilot light.

A gas water heater nearby works, so gas is available.

Same thing happened one night last year, and the next morning came on inexplicably when I fiddled with the thermostat a little. Any ideas?

Does the inducer start up?
If so, Does the ignitor engage?
If so, Does the unite fire for a few seconds then shut off?

To answer: inducer seemed to start and run for about 10 minutes. Not sure about ignitor, but the noise level increased as if furnace was "working", but air blowing out of vents is weak and not warm. Noise level has been constant since it increased the first time.

Maybe 20-30 minutes later, I think, the inducer started again and ran for 10 minutes before the louder sound kicked in. Still no heat. Should I be able to see a flame anywhere? (I don't.)

I noticed an LED indicator flashing a 31 code = "Pressure, draft safeguard, aux-limit switch, or blocked vent switch did not close or reopened".

Could be one of a couple of different issues. Inducer could be failing, pressure switch could be on it's way out, flue could be bocked etc. etc.

GEEWIZ ---Does anyone EVER answer anything on these sites?? When I got a 31 code -a Tech asked for a bucket- and SHOWED me how he disconnected this HOSE on the left of the fan motor from this WHITE plastic rectangle box(called a condensate trap) and a GALLON of water drained out. That was 2 years ago in 2009- today the same code popped up and I did the same thing-- it cost me $240 last time and $0 this time. I'm looking for a replacement trap part since it may just need replacing and looks like a DIY issue.