Duo Therm Mod# 90120.001 runs but won't light

There are only three things that stop a burner from lighting (or engine from running): 1. No fuel 2. bad air/fuel mixture 3. no source of ignition.

Guess what? It's one of these three.

For starters, I actually have this furnace. First things first, make sure you have sufficient voltage to the unit. It runs on the 12 volt system, so make sure the battery is fully charged and/or you are plugged in to "shore" power and your inverter is working. If the voltage is low, the fan will run, the relays will click, but the gas valve may not open and the igniter may not have the juice to run.

At startup, you should hear relays clicking. Three times, on/off, 10 seconds apart, before the board gives up trying. If you don't hear this, probably bad board or bad hearing, but possibly bad sail switch (switch tripped by main blower air flow), relays, or gas valve actuators.

The blower will run even with the control board disconnected.

Next, check for obstructions in the inlet/exhaust, like wasp nests. Wasps LOVE hydrocarbons for some reason. Truthfully though, this shouldn't be a problem unless the nest completely obstructs the tube. This furnace has a choke which restricts the airflow to an area about the diameter of a large middle finger or a small thumb anyway. That's what the "burner air adjustment" adjusts.

If everything is clicking but it still doesn't light, you should be able to smell gas at the exhaust. If not, guess what? Probably no gas. Points to bad board, sail switch, low voltage at valve, or bad valve (unlikely).

Assuming you have gas smell, check the connection to the igniter. Heat/cool-down cycles will tend to loosen the nut. It's a .25 inch socket by the way. To get at it you need to remove the furnace from the RV.

After removal, to get the furnace out of its shroud, you only need to remove the hex screw at the center-front by the gas valve and the wiring pull-thru on the right side. Then the unit will slide out.

While you're at it, check the gas orifice for obstructions. I did this by removing the air inlet shroud, but there may be an easier way. I think you can remove the whole gas assembly instead, but removing the shroud lets you clean everything up a bit. I had mud wasps obstructing the inlet tube. The tube has a choke inside like a flattened golf ball. If you remove the tube it will fall out. It's a little tricky to get back in, but not impossible. Just line it up with a nail.

The gas jet is quite large as gas jets go. Remove the jet and clean it with brake cleaner and/or compressed air.

Check all connections, tighten as necessary. Clean and tighten ground connections, blow or vacuum away dust. Vacuum through the exhaust while you are at it.

Reassemble and spark her up on the bench.

You can McGyver a battery connection to the unit to test operation. Red to red, negative to yellow. The blue wires are for thermostat. Short them together to test operation.

This won't help much without gas, but you can at least see that the sail switch operates properly.

The sail switch controls gas flow.

You can't really test the igniter easily. It's high voltage and a pig to remove and replace. If you don't have a spare gasket, don't.

You can check voltage to gas valve. If none, it is either sail switch or board. Jump sail switch to test which.

Now the iffy bit. If the voltage is lower than 9.5 volts, the valve probably won't open. The question becomes "why." Bad ground? Bad connection somewhere? Bad input voltage? Bad board?

Check, clean and tighten connections. Clean edge connector on board - gently with cloth or pencil eraser. Try again.

This is the situation I have; there is a voltage drop at the valve (between connector and ground) of 3 volts. The furnace runs on shore power, but not on a fully-charged battery. I've ordered a new board.

If you don't understand me, or it still doesn't work, you need a licensed tech with access to parts. He should be able to bench-test unit, so just slide it out and take it to him.

Good luck!

PS: New board from http://www.dinosaurelectronics.com/ did the trick.