Suburban Furnace: “High-limit lockout”
warning RV furnace fault codes
What it means
The high-temperature limit switch is a normally-closed safety in series with the ignition circuit. If the furnace overheats — typically from insufficient airflow — the limit opens and cuts the gas valve. On the 12 VDC Fan Control board, if the limit circuit stays open for 5 minutes during a heating cycle, the board enters lockout and shuts down the blower, and the thermostat must be reset to run again. Lesser overheating shows up as the furnace repeatedly 'cycling on limit' (flame cutting out and relighting).
Most likely causes
- Insufficient or blocked return air (covered/obstructed return grille) most likely
- Restricted, kinked, too-small, or sharply bent supply ducting raising plenum static pressure common
- Loose or leaking duct collar connections letting hot air escape and overheating the chamber common
- Dirty blower wheel reducing circulating airflow less common
- Failed/open limit switch or its wiring less common
Safe checks you can do yourself
- Make sure the return-air grille is fully open and not blocked by bedding, storage, or furniture.
- Confirm supply registers are open and ducts are not crushed, kinked, or closed off.
- Check that no register is buried in a closet or cargo area that cannot recirculate air back to the furnace.
- Note the symptom: short-cycling flame with the blower still running suggests cycling on limit from airflow restriction.
How to reset / clear High-limit lockout
- Clear whatever is restricting airflow (open grilles/registers, straighten ducts).
- Turn the thermostat OFF and let the furnace cool and the blower stop.
- Wait at least a minute, then turn the thermostat back up to restart; if it locks out on limit again, do not keep resetting — fix the airflow restriction first.
When to call a technician
If return air and ducting are confirmed adequate but the furnace still overheats, cycles on limit, or locks out on the limit circuit, have a technician test the limit switch and inspect the heat exchanger for cracks.
Applies to
SF-20F · SF-25F · SF-30F · SF-35F · SF-42F · NT-12S · NT-16S · NT-16SE · NT-20S · NT-24SP · NT-30SP · NT-34SP · NT-42T
FAQ
What does Suburban Furnace code "High-limit lockout" mean?
The high-temperature limit switch is a normally-closed safety in series with the ignition circuit. If the furnace overheats — typically from insufficient airflow — the limit opens and cuts the gas valve. On the 12 VDC Fan Control board, if the limit circuit stays open for 5 minutes during a heating cycle, the board enters lockout and shuts down the blower, and the thermostat must be reset to run again. Lesser overheating shows up as the furnace repeatedly 'cycling on limit' (flame cutting out and relighting).
How do I reset Suburban Furnace High-limit lockout?
Clear whatever is restricting airflow (open grilles/registers, straighten ducts). Turn the thermostat OFF and let the furnace cool and the blower stop. Wait at least a minute, then turn the thermostat back up to restart; if it locks out on limit again, do not keep resetting — fix the airflow restriction first.
What is the most common cause of High-limit lockout on a Suburban Furnace?
The most likely cause is insufficient or blocked return air (covered/obstructed return grille).
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Sources
- Suburban Furnace Service & Training Manual (SF 20/25/30/35/42 series) — Sequence of Operation (12 VDC board) — limit open 5 min = lockout; Return Air & Ducting sections (cycling on limit) (Suburban Manufacturing Company)
- Suburban DYNATRAIL Furnaces NT-Series Service Manual — Sequence of Normal Operation (limit switch in series to module board); Return Air / Ducting requirements (Suburban Manufacturing Company)
Note: The 5-minute limit-open-to-lockout timing is specified for the 12 VDC Fan Control board. On the 24 VAC park-model board, an open limit closes the gas valve but the blower keeps running rather than full lockout, so behavior differs by board.
● Last verified 2026-06-29 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual.