My parents' furnace burns diesel. The flame sensor (thermocouple) sometimes gets covered in soot and won't allow the furnace to light off.

LOL.

My brother and I just opened a heating and cooling company. It'* his trade, yet I still know a lot about HVAC.

Anyway. How old is the furnace and who is the manfucaturer? Any idea of the age?

What is it doing/not doing? Be as descriptive as possible.

in gonig to assume natural gas, it which case it would be piped in, other wise he would/should have said oil or propain

Even a propane-fired furnace will have the 1/2" gas line piped into the gas valve.

Just like working on cars, you need to tell us all the symtoms. Sure, it could be one of about 25 different things....

Oooh a pro :P

Its an Armstrong Ultra SX90 Gas-Fired Furnace
I went down and took the side panel off the cabinent, and it will start up, and i can see flame in the little window thing, and like not even 30 seconds later it will shut off, it doesn't stay on long enough to heat the house.
As for Age, i'm not 100% sure, it was in the house when my mom bought it, and she bought it in 2000, so its at least 8 years old and its probably a lot older then that. And my mom is in Chicago right now, and she hides all the paper work for everything so i wouldn't be able to find an install date or anything.
And yes its natural-gas

Turn off the gas and open up the burner area. Look at the condition of the thermocouple.

This is hilarious. We just had a call last night for the EXACT same symptoms on the exact same model furnace. It'* likely the gas valve. But there are things you can check before coming to that conclusion. Clean the pilot real well with some sand cloth. When you have the pilot assembly off (on the right side of the furnace, just in front of the burners where they aim into the heat exchanger), take a needle and and make sure there is no dirt in the orfice. It'* likely clean already, but check it anyway.

Then, if you're handy enough, remove the burner. You'll have to swing it out of the way since it'* piped into the outlet side of the gas valve. Do the same thing with each of the orfices in the four burners.

Reassemble everything. Try everythign again. If it'* no better, then your gas valve is the culprit. Those Armstrong units are notorious for having gas valves that will shut off the pilot gas supply before opening the main line to supply gas to the burners. The flame sensor senses no flame so shuts off the gas valve from pouring gas into the heat exchanger.

The Armstrong Ultra SX90 does not have a pilot. It uses a sparker I beleive. There are a few things that will shut down the ignited gas once its lit. the most common is the Thermocouple/Flame sensor. Your description of the problem fits this perfectly and I would just replace it and see if it fixes it.

Basically your furnace is saying "hey I've been pumping natural gas into this furnance for 30 seconds and I don't sense a flame. I better stop now before I blow the house up" A wise decision. Since you saw you see the flame then its definitely not the ignitor. Also, a thermocouple is alot cheaper than a new gas valve. But hey, its your money.

The flame sensor is not suspect. Clean the pilot (AKA electronic ignitor) and see where that leaves you (with the needle through the orfice as I already posted). Start with the free stuff first. But I've got $10 on the fact that you have a bad gas valve. The cost from the supply house is $130. Markup from your local heating man will price it out to about about $250...plus your installation costs.

If you dare to clean the flame sensor, be real careful. They've very brittle and it will likely break when you attempt to clean it.