Furnace will not run - Lennox SL280UHV checklist

What is going on

SL280UHV / SL280UHNV furnace does not start on a heat call; basic checks from the installation manual before deeper diagnosis.

Likely causes

• No thermostat call
• Panels off (door interlock open)
• Power off, blown fuse or tripped breaker (incl. 3 A 24 V breaker)
• Gas off
• Dirty filter tripping limit
• Lockout
• Pressure switch open (blocked flue)
• Flame rollout switch tripped

Check in this order

1. Confirm the thermostat is calling for heat.
2. Make sure the access panels are on (door interlock switch).
3. Check the main disconnect, fuses/breakers, and the 24 V circuit breaker on the control box (3 A, push to reset).
4. Make sure gas is on at the meter, the main manual shut-off and the internal gas valve switch.
5. Check the filter - a dirty filter makes the limit shut the unit off.
6. Check the 7-segment display for an error code (hold the button for 'E' to recall the last 10 codes). Watchguard lockout resets after one hour.
7. Check the pressure switches and flue/vent outlet for blockage.
8. Warning: If a flame rollout switch has tripped, find the cause (combustion air, blocked vent) before resetting it.

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