Single-stage 80% furnace: ignition sequence and retries

What happens, in order

1. Thermostat closes R-W to call for heat
2. Control runs a self-check and confirms the pressure switch is open, then starts the inducer
3. Inducer reaches speed, pressure switch closes, and the prepurge timer starts (humidifier 24 V terminal powers up)
4. Hot surface igniter warms up after prepurge
5. Gas valve opens at the end of warm-up; flame proving window starts 5 s later and lasts 2 s
6. Igniter stays on until flame is sensed or the 2 s proving window begins
7. Flame proven at the sensor gas valve stays open and the blower-on delay starts
8. No flame within 2 s gas valve closes and the control repeats the sequence up to 3 more times (4 trials total)
9. After the last failed trial the control goes to ignition lockout; it clears by itself after 3 hours, or by cycling 115 V or 24 V power to the control (not at W, G or R)
10. Flame seen when it should be off control locks out gas heat and keeps the inducer running until flame is gone

Timings and thresholds

Codes you may see during this

13 14 21 22 23 45

Models

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