I have a carrier gas furnace with electric ignition, the igniter comes on, then the flame kicks on, but goes out after about 5 seconds then it tries again about 20 seconds later. I would say its probably the flame sensor, since when i unplug the sensor, it does the exact same thing. Is there anything else that I am overlooking that could be causing the problem?

i think you have a control valve problem and you need to call a service man because the gas is something you should not mess with unless you know what you're doing. it can go blue on you and then no more house.

I have a two stage high efficiency Carrier furnace installed in 2000 that we've been having trouble with every winter. The furnace will intermittently stop running and won't come on unless the front cover to the furnace is pushed or the thermostat is clicked off and then on (the latter doesn't always work though and we have to go push the door anyway). We change the air filters every month, and even had the thermostat (programmable) changed twice, the last time by a service tech. Nothing seems to work. The manufacturer said that since the furnace kicks on when the door is pushed or the thermostat is turned off and then on, its a thermostat problem. The manufacturer of the thermostat (Honeywell) says that the thermostat is working properly and that its the furnace that is the issue. Anyone got any ideas to fix this furnace since these two companies aren't being any help? Thanks in advance!

If it's the switch behind the blower-compartment door, you can run a simple check by temporarily bypassing the switch and leaving it out of the circuit. But, from what you say, your intermittent defect may not be the switch. If proper operation is restored by switching the thermostat off and on, that is an indication of a lockout condition - which occurs after the computer detects an operational problem. Your furnace probably has a diagnostic LED light that normally indicates an area that needs to be investigated when the computer locks out. The HVAC tech should focus on pressure switches, loose connections, and resettable limit switches. Your frustration is understandable. Intermittents are nightmares to the techs also. They don't want to be your house guest for however many weeks it takes the furnace to malfunction again. The next time this occurs do nothing. Call a heating company immediately. If you fiddle with the thermostat or furnace you may reset the CPU. In doing so you will erase the trouble code, thus denying vital information to the tech. Good luck.