Furnace Starting Question

[Rob_Lowe]

I have a ~ 12 year old Carrier gas furnace. Most recently (at least it occured at the end of the heating) it has failed. It will begin to start, then ~40 seconds later automatically shut down. It does not sound like there is ever any combustion (usually a noticeable sound).

Maybe 5 years ago, while I was on a trip, a similar situation occured. Then my wife made a service call, where the repair guy replaced the Ignitor.

Is there a way I can Now verify that the ignitor (~$22 part) has again failed?

Might there be other likely causes, for our Failure to Start situation?

[Arthur Conan Doyle]

There are usualy some LEDs on the control board that indicate what caused the failure. The ignition sequence is pretty standard for most modern gas furnaces. The flue fan comes on and causes a vacuum switch to trip. That initiates the gas valve opening and ignigtor power. If the flame switch doesn't trip within a certain amount of time, the gas valve closes and the furnace shuts down. If the flame trips, then the blower starts.

Some furnaces have oxygen sensors as well. Sometimes you can sand a dirty flame switch, but some of them can be damaged by sanding, so they have to be replaced.