Dometic RV Furnace Runs 10 Seconds Then Shuts Off - Complete Troubleshooting Guide

The Quick Answer

CRITICAL SAFETY WARNING: Before performing any work on gas appliances, shut off the main gas supply and allow the system to purge completely. Work only in well-ventilated areas and have a gas leak detector available. If you smell gas at any time, stop work immediately and contact a professional.

Your Dometic DFMD35111 furnace is likely shutting down due to flame sensing issues or incorrect gas pressure, despite all the excellent work you've already done replacing the sail switch, igniter assembly, gas valve, control board, and temp sensor. Since you've eliminated the most common causes, common causes likely include either a faulty flame sensor (different from the temp sensor), improper gas pressure at the orifice, or a grounding issue with the control board. Given that you were "rooked" for $300 by a tech who clearly didn't fix it, let's get this sorted with some targeted diagnostics.

What's Causing This

Your 10-second runtime pattern is classic flame sensing failure. The furnace ignites successfully (proving your new igniter works), but the control board isn't detecting the flame properly, so it shuts down for safety. Even though you replaced the control board, there are three overlooked culprits that commonly cause this exact symptom.

First, the flame sensor rod itself may be corroded or positioned incorrectly. This is a separate component from the temp sensor you replaced - it's a small metal rod that sits in the flame path and sends a microamp signal back to the control board. Second, your gas pressure might be incorrect at the new orifice, causing an unstable flame that the sensor can't reliably detect. Finally, poor grounding between the control board and furnace housing can cause erratic flame sensing signals.

The fact that your furnace ran fine before and you've systematically replaced quality components suggests this isn't a design flaw but rather one specific component or connection causing the flame sensing circuit to fail. The $300 tech probably didn't check these items because they require actual diagnostic skills rather than just parts swapping.

Tools You'll Need

• Digital multimeter with microamp capability
• Manometer or gas pressure gauge (0-15 inches water column)
• Fine steel wool or emery cloth
• Wire brush
• Electrical contact cleaner
• Basic hand tools (screwdrivers, wrenches)
• Flashlight or headlamp for illumination
• Wire strippers and electrical tape
• Gas leak detector or soapy water solution

Step-by-Step Fix

WARNING: Shut off main gas supply before beginning any work. Test all connections for leaks before restoring gas service.

Start by locating and inspecting the flame sensor rod - it's typically a thin metal rod positioned near the burner assembly, separate from your replaced temp sensor. Remove it carefully and clean it thoroughly with fine steel wool until it's bright and shiny. Even microscopic corrosion can prevent proper flame detection. Check that it's positioned correctly in the flame path according to your manual specifications.

Next, test your gas pressure at the inlet to your new gas valve using a manometer. The DFMD35111 typically requires 11 inches water column gas pressure (verify exact specifications in your unit's manual or with manufacturer). If pressure readings appear significantly off, you may have a regulator or supply line issue that's causing flame instability. Also verify that your new gas orifice is the correct part for your model (verify with dealer) - even the right part number can sometimes be wrong due to manufacturing errors.

Check all ground connections between the control board and furnace housing. Remove, clean with contact cleaner, and retighten every ground screw you can find. A poor ground can cause the flame sensing circuit to behave erratically. Pay special attention to the control board mounting screws and any ground wires attached to the gas valve.

CAUTION: The following diagnostic requires the system to be operational with gas flowing. Ensure all electrical connections are secure before lighting. If you are not comfortable with live gas system diagnostics or working with electrical connections while gas is flowing, stop here and contact a qualified technician.

Finally, with everything cleaned and reconnected, test the flame sensor circuit with your multimeter. Set it to measure microamps and connect it in parallel across the flame sensor circuit (positive lead to flame sensor wire, negative to ground) while the furnace is running. You should typically see 2-10 microamps when the flame is present (verify specifications with manufacturer documentation). If you see zero or fluctuating readings, you've found your problem - either the sensor itself or the control board's sensing circuit may be faulty.

When to Call a Pro

Call a different, more qualified technician if your gas pressure readings are significantly off - this indicates regulator or supply line problems that can be dangerous to tackle yourself. Also seek professional help if you're uncomfortable working with gas connections, if you detect any gas leaks during your work, or if your multimeter shows proper flame sensing current but the furnace still shuts down.

However, given your systematic troubleshooting approach and the fact you've already replaced most components correctly, you're probably capable of completing the basic inspection and cleaning tasks yourself. The flame sensor cleaning and ground connection checks are straightforward tasks that don't involve gas work. If these steps don't solve it, the issue may be in the control board's flame sensing circuit - even new boards can be defective, and you might need to warranty replace the control board you just installed.