Furnace sequence

Learn the order things happen and you'll know exactly where it's stopping.

The short version

Thermostat calls, then the blower starts.
Blower gets up to speed and closes the sail switch.
Short purge, then spark and the gas valve opens.
Flame is sensed and it keeps running. If not, it tries again, then locks out.
Blower runs but nothing else happens? Usually low voltage (slow fan), a blocked return, or the sail switch.
Lights, then shuts off after a few seconds: flame sensing. Check the electrode, its wire and the board ground.
Runs, shuts the burner off early and the blower keeps going: the high limit is tripping. Look for restricted ducts or closed registers.
Locked out: most boards retry a few times, then stop. Cycle the thermostat or power to reset after you fix the cause.

Thermostat up 5 degrees. Stand at the furnace and listen: blower, then click and whoosh 15 to 30 seconds later.
No blower: 12V on the thermostat wire at the furnace, then the fuse, then 12V at the motor leads with the call on.
Blower but no spark: meter across the sail switch with the blower running (near 0 V closed). Check voltage at the furnace under load.
Spark but no flame: light the stove, then manometer at the valve test port during the trial (11 in WC), then 12V at the valve coil.
Lights then dies: clean the sense electrode, set the gap to about 1/8 in, ohm burner to board ground (under 1 Ω).
Short cycling: open two registers, clear the return, remove any filter, check the blower wheel. Then test the limit.

Most "no heat, fan runs" calls in winter are a weak battery. Check voltage at the furnace with the blower going.