1 flash

Atwood Furnace: “1 flash”

warning RV furnace fault codes

Safety This is a propane combustion appliance. Never operate it with blocked exhaust or intake vents, which can cause overheating or unsafe combustion. If you smell gas, shut off the propane at the tank, ventilate, do not use switches or flames, and leave the RV before calling a technician.

What it means

One LED flash followed by a 3-second pause, repeating. Indicates a limit switch / airflow problem: the board is not seeing the conditions it expects for safe airflow/temperature. This is a soft lockout, so the furnace will keep retrying once the underlying airflow/limit condition clears.

Most likely causes

Blocked or restricted airflow (blocked return-air opening, closed/crushed heat ducts, blocked exhaust/intake vent, debris in blower) most likely
Limit switch (overheat safety) tripped open due to overheating common
Weak or failing blower motor / low blower speed from low voltage common
Faulty limit switch or its wiring less common

Safe checks you can do yourself

Make sure nothing blocks the return-air grille and that supply registers/ducts are open and unobstructed.
Check the exterior intake/exhaust vent for blockage (nests, snow, debris) and clear it.
Listen to the blower: it should spin up strongly. Verify a healthy 12V supply, since low voltage slows the blower and can trip the limit.
Let the furnace cool fully, then call for heat again; a one-time overheat trip may clear on its own once airflow is restored.

How to reset / clear 1 flash

Clear the airflow restriction or let the furnace cool so the limit switch resets.
Turn the thermostat off and the setpoint all the way down; wait for the blower to stop.
Turn the thermostat back on and raise the setpoint. If it ran into a hard condition, also cycle 12V power off for ~30 seconds before restarting.

When to call a technician

Call a technician if the 1-flash code persists after you have confirmed clear airflow, clear vents, a healthy battery/voltage, and a working blower, since the limit switch or blower motor may be faulty and a repeatedly tripping overheat limit indicates a real safety problem.

Applies to

AFMD16 · AFMD20 · AFMD25 · AFMD30 · AFMD35 · Atwood Hydroflame AFM/AFMD series furnaces with the standard 3-tries-for-ignition diagnostic control board

FAQ

What does Atwood Furnace code "1 flash" mean?

How do I reset Atwood Furnace 1 flash?

Clear the airflow restriction or let the furnace cool so the limit switch resets. Turn the thermostat off and the setpoint all the way down; wait for the blower to stop. Turn the thermostat back on and raise the setpoint. If it ran into a hard condition, also cycle 12V power off for ~30 seconds before restarting.

What is the most common cause of 1 flash on a Atwood Furnace?

The most likely cause is blocked or restricted airflow (blocked return-air opening, closed/crushed heat ducts, blocked exhaust/intake vent, debris in blower).

Related · Atwood furnace

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Sources

Dometic (Atwood) Furnace Model AFM, AFMD Series Operating Instructions — Ignition Control Diagnostic Codes chart: 'Limit Switch/Airflow Problems - 1-Flash With 3-Second Pause - Soft' (Forest River, Inc. (OEM-hosted manufacturer document))
Atwood Furnace AFM Operating/Service Manual — Diagnostic chart: Limit Switch/Airflow Problems = 1 flash with 3-second pause, Soft lockout (Atwood Mobile Products (Heartland Owners manuals library))

Note: Applies to the single-stage standard ignition control board. The fault groups 'limit switch' and 'airflow' together, so the code does not by itself tell you which one; airflow restriction and overheat trips are the usual first things to check.

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