I am looking at furnace for a friend. The furnace will come on with that small fan and the pilot will light. However, the burner will not catch.

This unit has a igniter I suspect that the igniter is bad. Is there any way to test it?

labman

You should be able to see it get red hot. also, if the gas comes on, but goes off because it didn't light, it isn't working.

Disregard above answer. I was in a hurry and didn't read the the question correctly.

skiberger

when the fan is coming on ,pre-purging, its making sure there is no left over "exhaust" in the system and no back drafting. If the pilot is lit, that is your ignition source. It almost sound like there is a saftey "switch"preventing the gas valve from opening on the pre-purge cycle. If the furnace vents throught the side of the house and not a chimney, check to see if the exterior of the vent is blocked by birds, bees nest, etc. Also make sure the bottom compartment door cover the blower is securly on. (theres a saftey switch on the compartment that will prevent the furnace from igniting).
Just some more things to check out.

pipwax

Thanks I will check.

labman

If you have a pilot light lit, and the main burner doesn't light, the gas valve isn't opening. Check to see if it is getting 24 volts. If not, follow the wiring back to where the white wire comes in from the thermostat.
There should also be likely a blue wire connecting the blower relays and the gas valve to one side of the transformer.

The small blower should be a forced draft and run the entire cycle.