Heat7 min
Gas furnace sequence
If you do not know the sequence you will replace a good board. Watch the LEDs and time it.
Typical induced-draft sequence
- Thermostat W. Inducer starts. Pressure switch closes.
- Hot surface ignitor warms (or spark). Gas valve opens. Flame sense proves.
- Blower starts after the OEM heat-on delay. On shutoff, gas valve first, then blower after heat-off delay.
Where it dies
- No inducer: 120 V at the inducer, door switch, board output.
- Inducer on, no ignition: pressure switch tubing, flue, switch ohms, ignitor amps / ohms.
- Ignition then drop: flame sensor (clean with a non-scratch pad, not sandpaper), polarity, ground, gas pressure.
- Limit trip: airflow, dirty filter, A-coil, overfire, or a failed limit.
Combustion
- Manifold pressure on high and low fire vs rating plate. Clock the meter if input is in doubt.
- CO in the flue and in the occupied space. If ambient CO is up, shut it down.