front furnance not working on 2004 vectra

Have you check voltage at the fan? What brand and model # of heater and thermostat do you have.

Several things have to happen for the furnace to "heat" but the sequence is for the thermostat to call for heat, the fan comes on (as long as you have 12V, sail switch makes and gas valve turns on.

Sounds like it could be the thermostat.

Is the pilot light on? It may have a function to not blow if the pilot is not lit.

Please let us know what brand furnace.

Likely there is no pilot light on that late a model furnace, but we are guessing without your information.

And, if you have nothing happening, yes, you need to verify 12 VDC at the furnace and also check the thermostat. If you have two furnaces with separate thermostats (see we don't know what you have) switch the known good thermostat for the suspect one.

We've had the same thing happen twice to our furnaces...it turned out to be the circuit board which in some cases is easy to pull out and replace without taking out the whole heating unit. It could be something else in your case though.

We called the manufacturer and they walked us through a troubleshoot routine to determine it was indeed the board. We then ordered a new board and we easily installed it. Even if it's not the computer board I'd call the manufacturer or a local dealer to see if they can determine what is wrong for sure.
Good luck.

The OP didn't give a lot of info, maybe I can jump in and help since I have as similar unit...

'04 Vectra has two types of heat... basement electric air/heat pump (Coleman Mach) and aux propane furnaces (front and rear.) The front propane furnace is likely a Suburban SH-42 (40k btu) and the rear likely a Suburban SF-25 (25k btu).

The air, electric heat, and front furnace are all controlled by the same thermostat, most likely a Coleman Mach 6535. The rear has it's own thermostat.

I'd suspect the thermostat, as that particular model has some issues per other threads. The slide switch tends to wear out, not making a good connections. There is a slightly upgraded version that is available and is an easy replacement. Went through it myself.

If you think that's your issue, see this link... replacement thermostat

Check your manual - 2004 Vectra, Page 8-10. The furnace should be a Suburban furnace, but don't know what model. Check the Suburban furnace page.

I had the same problem this year with my 04 itasca meridian. The suburban sh42 furnace would not do anything. It ended up being the circuit board. I got a replacement from dinosaur boards for just over $100 and the change out was easy once you remove the furnace for access. If yours is like mine the access is next to the hot water heater remove the screws that hold the cover plate in place and then disconnect the propane line and then 1 screw for the furnace and out it comes. unplug the wiring harness and move the furnace to a work bench.

I have the same problem. I looked at the wiring diagrams and found that the thermostat looks like it is ok. I got to the wiring in the cabinet to access the thermostat connections and there is 12V on the wires to the furnace. I connected the 12V wire to the thermostat line coming from the furnace, and nothing happened, so I assume that the furnace electronic module is the problem. My problem is that I don't know how to access the furnace. My RV is a 2004 Vectra Model AD, and it appears that the furnace is located above the plumbing bay on the drivers side, at least that is where the vent and exhaust is. I can't figure out how to get to it. Does anyone know how to get to the furnace?