Ignition lockout

Suburban Furnace: “Ignition lockout”

warning RV furnace fault codes

Safety Propane is explosive. If you smell gas, STOP, do not operate the furnace, ventilate, and if the smell continues shut the LP tanks and get help. NEVER jumper or bypass the sail switch, limit switch, or rollout switch to force the furnace to run — these prevent fire and carbon-monoxide hazards. Repeated lockouts can also flood the chamber with unburned gas, so let it purge between attempts.

What it means

Suburban NT and SF series RV furnaces do not display a numeric error code or LED blink code. After a heat call, the blower starts, the sail switch must close, then a ~15-second purge runs, then the board energizes the gas valve and sparks for about 7 seconds while watching for flame sense through the spark electrode. If no flame is proven, it repeats the purge-and-spark up to 3 total Trial-For-Ignition attempts. After the 3rd failure the board shuts the gas valve, runs the blower about 3 minutes to cool, and goes into lockout — the burner will not relight until the thermostat circuit is broken (reset).

Most likely causes

Out of propane, closed valve, or weak gas flow/pressure so the burner can't light most likely
Low 12V DC (board needs a minimum ~9.5V) causing weak spark or stand-by behavior common
Dirty, misaligned, or carbon-fouled spark/flame-sense electrode so flame is not detected (under 7 microamps) common
Sail switch not closing due to weak blower, dirty wheel, or blocked return-air/discharge airflow common
Failed gas valve, control (module) board, or open limit switch less common

Safe checks you can do yourself

Confirm propane is on and another LP appliance lights, proving gas flow to the coach.
Listen on a heat call: blower should start, then you should hear clicking (spark) after the purge. No blower at all points to 12V/fuse/thermostat; blower but no spark points to sail switch or board.
Check the furnace 12V fuse and that the RV battery voltage is healthy.
Make sure return-air grille and warm-air discharge registers are fully open and unobstructed so the sail switch can close.
Set the thermostat well above room temperature so it actually calls for heat.
Inspect the exterior furnace vent/intake for insect nests or debris.

How to reset / clear Ignition lockout

Turn the wall thermostat OFF (or turn the furnace's on/off switch off) for 30-60 seconds, then back to Heat with a higher setpoint — breaking the thermostat circuit clears the lockout.
If it won't reset, pull the furnace's 12V fuse (or its circuit breaker) for about one minute, reinstall, and retry heat to clear transient board faults.
If a limit-switch (overheat) lockout occurred, let the furnace cool fully before resetting the thermostat.

When to call a technician

Call a certified RV tech if the furnace still locks out after confirming gas, 12V power, clean airflow, and a clean/aligned electrode — or if the flame sensor reads at least 7 microamps yet the burner still drops into lockout (indicates the module board), or if you suspect the gas valve, limit switch, or sail switch.

Applies to

NT-12SE · NT-16SE · NT-20SE · NT-16SEQ · NT-20SEQ · NT-30SP · NT-34SP · NT-40 · SF-20 · SF-25 · SF-30 · SF-35 · SF-42 (and Q/F/FQ variants)

FAQ

What does Suburban Furnace code "Ignition lockout" mean?

How do I reset Suburban Furnace Ignition lockout?

Turn the wall thermostat OFF (or turn the furnace's on/off switch off) for 30-60 seconds, then back to Heat with a higher setpoint — breaking the thermostat circuit clears the lockout. If it won't reset, pull the furnace's 12V fuse (or its circuit breaker) for about one minute, reinstall, and retry heat to clear transient board faults. If a limit-switch (overheat) lockout occurred, let the furnace cool fully before resetting the thermostat.

What is the most common cause of Ignition lockout on a Suburban Furnace?

The most likely cause is out of propane, closed valve, or weak gas flow/pressure so the burner can't light.

Related · Suburban furnace

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The Fan Control module board continuously monitors supply vo… Sail switch lockout
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Sources

Suburban RV Furnaces Service & Training Manual (11-05-2015) — Sequence of Operation (12VDC / 24VAC Fan Control Module Board, pp. 26-29); Service Hints / flame-sense 7 microamp spec (p. 32) (Suburban Manufacturing (hosted by Heartland Owners Forum))
Suburban Furnace Service Manual (NT / SF / DD-17DSI series) — Sequence of operation and troubleshooting flow chart (Suburban (hosted by My RV Works, Inc.))

Note: Verified directly against the Suburban Service & Training Manual: 3 Trial-For-Ignition attempts, ~15-second purge, 7-second spark/flame-sense window, 7-microamp minimum flame current, and lockout cleared by breaking the thermostat circuit. Suburban furnaces give NO numeric display code or diagnostic blink code — diagnosis is by the sequence/symptom, so 'codeKind' is approximated as a lockout indicator. Exact timing (e.g., limit-switch open for 5 minutes vs. immediate) varies slightly by board generation (12VDC vs 24VAC vs time-delay-relay models).

● Last verified 2026-06-29 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual.