[snfexpress]

Is it actually lighting or are you hearing the click, click, click of the ignitor trying to light the gas?

We had a 2006 3400 where we detailed our experiences and fix for the problem:

quote:

There are just a few basic parts to these blasted furnaces.

First, there is the circuit board, which is the easiest to replace. Typically, if the fan is blowing and WON'T turn off unless you either pull the 12v fuse or turn off the unit on top (there is a switch), then it is probably the circuit board.

Second, there is a sail switch. This is a switch inside the fan housing (4 screws to get the fan housing front half off). This switch has a long piece of metal on it about 1/4" wide. As the fan turns, the air pushes against this long thin metal strip and pushes it closed, which activates the switch. The switch is now allowing 12v to go thru it which in turn allows the gas valve to be able to operate.

Third is the electrode. This is a bear to replace (I did it, and it took me an hour to get the old one out and the new one in by wiggling it every which way in a very cramped space). The electrode, if bad, won't "spark" and ignite the gas. If you go to the trouble of getting the unit all the way out and on a bench, then I would take the extra time and replace the electrode.

Fourth is the gas valve. I haven't replaced this valve, and if it ever gets to that point, I'm just going to order a new furnace. Looks too complicated and dangerous, at least to me.

So, in order of price, it goes 1, 4, 3, 2. Roughly $170 for the board and just a few bucks for the sail switch and electrode. Don't know the price of the gas valve.

In order of difficulty, it goes 1, 2, 3, 4.

In order of your problem, I would suggest #3. Let me ask you: Does you furnace ever light? The unit will try 3 times, then "lock out", so you have to turn the unit switch off then back on. When you turn it back on (with the t-stat turned to furnace and calling for heat - set to 99 degrees), the fan should come on, then the ignitor on the circuit board (after about 20 seconds) should go click, click, click and then the propane should be burning. If the propane does not ignite, the exhaust sound will not change, and then the board will try a second and a third time.

Oh, I would try one thing first. Open your stove hood and disconnect the gas to the burners. Fit a gas guage on to the propane supply line. It should read between 10 and 14 inches WC or about 3 lbs of pressure. But, Atwood's manual states that before doing a pressure check, to first disconnect the propane supply and cap this supply line to the furnace. If the pressure is low, go to your regulator and adjust as necessary (under the black cap on the regulator).

Sorry for the long post, and hope that what I wrote makes sense to you. Good Luck!