My furnace suddenly stopped working today. The thermostat triggers properly, and the inducer fan starts running, but furnace never fires and the blower doesn't come on. If I turn the blower on manually at the thermostat, it runs fine.

The furnace is an older Payne/Carrier natural gas/forced air unit. Maybe about 15 years old? Any ideas?

Thanks.

Pressure switch feed line plugged
Pressure switch defective
Bad glow coil/ignitor/spark system
Open limit control
Defective draft inducer end switch if so equipted.
Bad board if so equipped.

What you need to do is to go to the sequence of operations in the sticky at http://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html
Identify the last operation, and then check the sensor.

I'm getting to the point in the sequence of operations where the pilot ignites, but the main gas valve doesn't open. Would the most likely culprit be the flame sensor? Is there an easy/safe way to check the flame sensor?

You can try cleaning the flame sensor with snad paper or steel wood.

Is it possible the flame sensor is integrated with the igniter? There's a little integrated unit into which the pilot gas line connects, with the igniter wire on top (I can see it spark), and a switch of some kind on the bottom with three leads. One lead comes from the pressure switch. One lead goes to the main gas valve. One lead goes to an ignitor terminal (ground?). But I can't see anything else that looks like a standard flame sensor. I followed every wire and thoroughly scoped out the logical places where a flame sensor would be.

Thanks for the help.

Woohoo. Fixed it!

The problem was a dirty pilot burner. There was something in the gas tube (dead spider?) that was making the pilot flame too weak to trip the integrated sensor (there is no separate flame sensor. I took it apart, cleaned it, reinstalled it, and now I have heat and my family loves me. Thanks for the troubleshooting help. The image below is the pilot burner, in case anyone else runs into this problem with an old Payne Carrier forced air natural gas furnace.

http://i12.ebayimg.com/05/i/000/c1/d6/e2b0_1.JPG