Single- and two-stage 95% furnace: ignition sequence and retries

What happens, in order

1. Thermostat closes R-W (W1) to call for heat
2. Control self-checks and confirms the pressure switches are open, then starts the inducer (high speed on the two-stage)
3. When the low-heat pressure switch closes the 15 s prepurge starts and the 24 V humidifier terminal is powered (9MXE ignores the housing pressure switch until 1 minute after the gas valve closes)
4. Hot surface igniter warms for 17 s
5. Gas valve opens; flame proving starts 5 s later and lasts 2 s; igniter stays on until flame is sensed or the proving window starts
6. Flame proven inducer drops to low speed on the two-stage unit, and the blower-on delay begins
7. No flame in 2 s gas valve closes and the control repeats the sequence up to 3 more times before ignition lockout
8. Lockout clears automatically after 3 hours, or by interrupting 115 V power or 24 V at the control (not at W, G or R)
9. Flame seen when it should be off control locks out gas heating and runs the inducer (at high speed) until flame is gone

Timings and thresholds

Models

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