I Have a problem on a Coleman furnace, model # DGAA070BDTA. The problem is when the furnace kicks on, it lights the hot surface ignitor and lights the burner, but as soon as the fan comes on the burner shuts down and cycles about 5 times then shuts down. I have changed the board 2 times, replaced transformer, hot surface ignitor, flame sensor, thermostat, gas valve and pressure switch. When the furnace shuts down, it flashes a failed to ignite code. On the gas valve, I am getting 26.3 volts but when the fan comes on it drops to 0. Have worked on it for 3 days and am getting really frustrated. Any ideas?

The booster motor assembly has a built in end safety switch. When you say as soon as the fan comes on are you referring to the small booster/combustion air fan?

No, the blower fan. The small booster fan is fine. The minute the blower motor kicks on for some reason the voltage to the gas valve goes to zero and shuts the gas off.

I would trace your electric power wiring from the panel all the way to where it hooks up to the furnace. It seems like when you have a big power draw (like the blower motor) you loose power to the unit temporally allowing the low voltage to drop out causing the burner to cycle.

As you have found out all the parts you replaced were good so see if you can find the real fault. Also check any door switches if you unit has one.

One question. Does the blower motor run OK besides shutting down the furnace?

Yes the blower runs fine. Have not had any problems with that

You have a power problem somewhere! At this time I am out of resources. Unless someone else here has a idea I suggest you call for on site service. Usually is a service person is there it can be easier to track down.

OK thank you for all the help. I have one more thing to check tomorrow.

Check the yellow wire coming from the flame sensor, make sure it is not lying on the burner, may have a small burnt hole in wire.

Well humptydumpty, I found my problem the next day after my original post back in 2007. Found that some how a bird had built a nest in the roof vent and the circuit board detected the blockage and was shutting down the furnace the second the blower fan came on. That was over 3 yrs ago and still working.But thank you for your advice.