Model DGAA07BDTB
Green light is flashing, three times, indicating that the combustion air switch is stuck closed, how can I fix this problem so we can have heat? And where is the combustion air swich located? Thank you,

There is a pressure switch in the furnace which checks to be sure there is adequate draft through the furnace. It's kind of a silvery pancake about 3" in diameter, with one or two small pieces of rubber tubing connecting to it and two or three wires.

The ignition control board checks to see that the switch isn't stuck in the closed position as a safety check, and your pressure switch appears to be failing that check.

Use the handle of a table knife and gently tap of the pressure switch with the idea of jogging the stuck switch to encourage it to open. Don't pound on it, just some light to moderate tapping.

Switch the furnace on and see if that gets it started.

If it does, perhaps that's all it will need. If the problem recurs, you'd need to replace the pressure switch with the part number SPECIFIED BY THE MANUFACTURER FOR YOUR FURNACE. This last is very important!

Its not uncommon for a pressure switch to stick. With the power off to the system You can remove the rubber hose from the collector box or vent, with it connected to the pressure switch you can blow and suck lightly on the hose. You should hear a light click at the switch. Make sure the port is not restricted at the collector box or exhaust where you connect the rubber hose.

I recommend against blowing or sucking on pressure switches. That can rupture the rubber diaphram in the pressure switch, and it's a lot more likely to ruin the pressure switch setting which is critical to the safety of the furnace.

Key word lightly. But I have heard of people wanting to use force by beating on them to get it to work. Remember that a inducer motor applies a small amounts of pressure to the switch not blunt force.

My furnace starts about every 15 minutes, runs for hours when it is cold, what could the problem be

liseray,
Either the furnace is sized exactly to the heat loss (highly unlikely) or you have warm air going somewhere other than into the living space.