Suburban · SF and NT-series furnaces; confirm board legend
3 flashesIgnition lockout
On applicable control boards, three flashes indicate that ignition was attempted but a stable flame was not proven.
What it means
Plain-English answer
The furnace blower may run while the burner fails to light or remain lit. The exact flash legend on the installed board and model manual takes priority because board revisions vary.
Owner-safe checks
What to check first
- 01
Confirm adequate battery voltage and that the propane supply is on.
- 02
Check the exterior intake and exhaust for nests, snow, or debris without inserting tools.
- 03
After the blower stops fully, cycle the thermostat once.
Safety boundary
Stop and call a technician when
- You smell propane, see soot, or the exterior exhaust is damaged.
- The lockout returns; gas-pressure and flame-sense tests require qualified service.
Primary reference
Check the exact manual
Applicability: SF and NT-series furnaces; confirm board legend. The data plate and specification letter determine which procedure is correct.
Open Suburban furnace operation guide ↗Source reviewed August 2026