Why does my Dometic furnace blower run but the burner won't ignite?

This is commonly caused by a faulty ignition board, a dirty or failed sail switch, or insufficient propane supply. First, verify your propane tank is open and has fuel, then check that the sail switch is closing properly when the blower activates. If both are fine, the ignition control board may need to be replaced.

Can a weak RV battery cause my Dometic furnace not to ignite?

Yes, low battery voltage is one of the most common causes of ignition failure in Dometic furnaces. These furnaces require a minimum of 10.5 volts DC to operate properly, and the ignition board is especially sensitive to voltage drops. Check your battery voltage under load and ensure all connections are clean and tight.

How do I know if the sail switch is preventing my Dometic furnace from igniting?

The sail switch is a safety device that confirms adequate airflow before allowing ignition. If the switch is stuck open, dirty, or misadjusted, the furnace will blow air but never attempt to light. You can test it with a multimeter for continuity while the blower is running; if it doesn't close, it needs cleaning or replacement.

Could a blocked exhaust or intake vent stop my Dometic furnace from lighting?

Absolutely. Blocked intake or exhaust vents caused by insect nests, debris, or ice buildup can prevent proper combustion airflow and cause the furnace to lock out. Inspect both the exterior intake and exhaust tubes thoroughly and clear any obstructions. Mud dauber wasps and spiders are notorious for nesting inside RV furnace vent tubes during storage.

What should I check if my Dometic furnace sparks but won't stay lit?

If the furnace ignites briefly but shuts down, the flame sensor may be dirty or faulty and unable to detect the flame, causing the control board to shut off the gas valve. Remove the flame sensor and clean it gently with fine steel wool or emery cloth. If cleaning doesn't resolve the issue, also verify consistent propane pressure and check the gas valve for proper operation.

Dometic Furnace Won't Ignite — RV Diagnostic Guide

Probable Causes

Empty or low propane tank — the most common and most overlooked cause
Closed or partially closed LP gas valve (tank valve, manifold valve, or furnace gas valve)
Dead or weak 12V house battery — the furnace blower and control board need solid 12V DC to operate
Dirty or failed sail switch — the switch that confirms the blower is running before gas is released
Faulty ignition board (DSI module) — the circuit board that controls the ignition sequence
Corroded or cracked electrode — produces weak or no spark
Blocked exhaust or intake vent — mud dauber nests, leaves, or debris preventing proper airflow
Failed gas valve solenoid — valve doesn't open even when commanded
Tripped high-limit switch or thermal fuse — safety device triggered by past overheating

Urgency & Safety

High priority. Any LP gas issue carries a risk of gas accumulation. If you smell propane at any point, shut off the tank immediately, ventilate the RV, and do not operate electrical switches. Never bypass safety devices. Carbon monoxide is also a concern — ensure your CO detector is functional before troubleshooting.

DIY Difficulty

Moderate. Basic checks (propane, battery, vents) are beginner-level. Anything involving the gas valve, ignition board, or electrode replacement is intermediate to advanced and may require specific tools and LP gas leak-detection solution.

Typical Repair Cost

| Repair | Estimated Cost | |---|---| | Propane refill | $20–$50 | | Sail switch replacement | $15–$40 (part) | | Ignition control board | $80–$200 (part) | | Electrode assembly | $20–$50 (part) | | Gas valve replacement | $80–$150 (part) | | Professional labor (shop) | $100–$175/hr |

Parts You May Need

Replacement sail switch (model-specific to your Dometic unit)
DSI ignition control board
Electrode/igniter assembly with gasket
High-limit switch / thermal fuse
LP gas leak-detection solution or spray

Step-by-Step Diagnosis

Check your propane supply. Verify the tank is not empty. Open the tank valve fully. If you have other LP appliances (stove, water heater), try lighting one to confirm gas flow. If the stove won't light either, the problem is upstream — tank, regulator, or a closed valve.

Check your 12V battery voltage. Measure at the house battery terminals with a multimeter. You need at least 10.5V under load, but ideally 12.4V+. A weak battery can power the blower motor sluggishly, preventing the sail switch from closing.

Inspect exterior intake and exhaust vents. Go outside and locate the furnace vents (typically two openings side by side on the sidewall). Look for insect nests, dirt, or obstructions. Clear any debris. A blocked vent will cause the furnace to lock out.

Listen to the startup sequence. Set the thermostat above ambient temperature. You should hear: (a) the blower motor spin up, (b) a clicking sound (igniter sparking), and (c) a "whomp" of ignition. Note where the sequence stops — this tells you what failed.

No blower at all? Check the furnace fuse (usually on the board or inline), the thermostat wiring, and 12V power at the furnace connector.

Blower runs but no clicking? The sail switch likely isn't closing, or the control board isn't sending spark. Inspect the sail switch by gently blowing into the intake — you should hear it click. Test continuity with a multimeter. Replace if open.

Clicking but no ignition? Inspect the electrode for cracks, heavy carbon buildup, or an incorrect gap (typically 1/8"). Clean or replace. Also verify the gas valve is receiving voltage during the ignition attempt (~12V DC at the valve terminals).

Ignites briefly then shuts down? The flame sensor function (handled by the same electrode on most Dometic units) may not be detecting flame. Clean the electrode tip with fine emery cloth. Also re-check for exhaust blockage.

Reset the lockout. Turn the thermostat off for 60 seconds, then retry. Most Dometic boards allow three ignition attempts before locking out.

When to Call a Technician

Call a certified RV or LP gas technician if you smell gas and cannot identify the source, if the gas valve needs replacement, if the control board has visible burn marks, or if the furnace repeatedly locks out after you've addressed the basics. Any repair involving LP gas line connections should be pressure-tested by a professional.

Dometic Furnace Won't Ignite

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