Dometic RV Furnace Flame Lights Then Dies: Complete Fix Guide 2024

The Rundown

David, your 2019 Dometic DFMD35111 furnace is experiencing intermittent flame sensor issues, which is causing the short cycling behavior you're describing. Despite your thorough troubleshooting - cleaning the burner, replacing the limit switch, adjusting the igniter gap, and checking the sail switch - the flame sensor is likely contaminated or failing. This is the most common cause of flames lighting immediately but then extinguishing after 2-3 seconds to several minutes, followed by successful relighting attempts. The fact that your LED control board isn't showing codes and the burner relights consistently points directly to flame sensor contamination rather than gas supply, ignition, or airflow problems.

Your systematic approach has eliminated most other potential causes. The varying burn times (5 seconds to 6 minutes) are classic symptoms of an intermittent flame sensor issue where carbon buildup or oxidation creates inconsistent electrical continuity. The sensor may work temporarily when clean but fails as it heats up and expands, breaking the connection needed to keep the gas valve open.

Why It Happens

The flame sensor in your DFMD35111 is a small metallic rod that sits in the flame path and conducts a tiny electrical current through the ionized flame. This current signals the control board to keep the gas valve open. Over time, even with minimal use like your RV had before you purchased it, the sensor develops a thin coating of carbon deposits, metal oxides, or other combustion byproducts that interfere with proper electrical conductivity.

What makes this particularly tricky is that the contamination can be inconsistent - sometimes the sensor works fine for minutes, other times it fails within seconds. Temperature cycling, humidity exposure during storage, and even the specific gas mixture can affect how quickly this buildup occurs. Your furnace sitting unused for periods actually makes this more likely, as moisture and oxidation can occur during dormant periods.

The delay you're hearing is the DFMD35111's safety lockout period - it waits before attempting to relight to prevent gas buildup. The immediate successful relighting confirms your ignition system, gas pressure, and airflow are all functioning properly. This pattern of quick relight followed by inconsistent burn duration is textbook flame sensor contamination.

Gather These Items

For this repair, you'll need fine-grit emery cloth (220-400 grit), a small wire brush, electrical contact cleaner or denatured alcohol, clean lint-free rags, and a basic multimeter if you want to test continuity. The flame sensor on your DFMD35111 is located near the burner assembly and looks like a thin metal rod extending into the flame area - you've likely seen it during your previous work.

You might also want to have a replacement flame sensor on hand (consult Dometic parts catalog for correct part number for DFMD35111 model, around $25-35) in case cleaning doesn't solve the issue completely. While you're at it, grab a can of compressed air for final cleaning, and consider having some gas-rated pipe thread sealant available if you need to disconnect any gas fittings, though hopefully you can access the sensor without major disassembly.

The Fix

Start by shutting off the electrical power to the furnace and the gas supply, then allow the furnace to cool completely. Access the flame sensor through your existing exterior panel - it should be visible as a thin metal rod positioned to extend into the flame area near the burner. Carefully disconnect the wire leading to the flame sensor, noting its position for reassembly.

Remove the flame sensor by unscrewing it from its mounting bracket. The sensor itself is just a metal rod, typically stainless steel (see manufacturer specs for exact diameter). Using your fine emery cloth, gently polish the entire length of the sensor rod, paying special attention to the tip that sits in the flame. You want to remove any discoloration, oxidation, or carbon buildup until the metal appears bright and clean.

Clean the threaded mounting area and electrical connection point with contact cleaner or alcohol. If you have a multimeter, test for continuity between the sensor rod and its electrical connection - you should see very low resistance. Reinstall the sensor, ensuring it's positioned correctly in the flame path, and reconnect the electrical wire. Make sure all connections are tight and properly seated.

Before testing, double-check that your igniter gap is still at the manufacturer's specification as you set it previously. Turn the gas back on and test the furnace. Ensure adequate ventilation during testing and operation. You should now see consistent operation with the flame staying lit continuously. If the problem persists after cleaning, the sensor itself has likely failed internally and needs replacement.

Know Your Limits

While flame sensor cleaning is a straightforward DIY repair, be aware that repeated failures after cleaning indicate sensor replacement is needed. If you're uncomfortable working around gas connections or if accessing the flame sensor requires disconnecting gas lines (which you mentioned was difficult on your unit), consider having an RV service technician handle this repair.

Never attempt to bypass or disable the flame sensor - it's a critical safety component that prevents dangerous gas accumulation. If cleaning and replacement don't resolve the issue, you may be looking at control board problems, which typically require professional diagnosis. Also, if you notice any gas odors, yellow flames instead of blue, or sooting around the burner area, stop immediately and call a professional - these indicate more serious safety concerns beyond a simple flame sensor issue.