Atwood AFMD35 Furnace Playing Peek-a-Boo - How to Clean Flame Sensor & Fix Short Cycling

Bottom Line Up Front

Your diagnosis is spot-on for your Atwood AFMD35 furnace. The pattern you're describing - fan runs, ignition occurs for 5-10 seconds, then shuts off, repeating three times before lockout - is the classic signature of a failed flame sensor. You're correct that sail switch or high limit issues would prevent ignition entirely. The flame sensor (part #31058) detects the flame presence and costs around $35-45. This is definitely a DIY-friendly repair that should take about 30-45 minutes once you gain access to the furnace compartment.

Root Causes

The flame sensor in your AFMD35 is a small metallic rod that sits in the flame path and generates a small DC current (microamps) when the flame creates a rectification effect between the sensor and ground. Over time, this sensor accumulates carbon deposits, corrosion, or simply degrades from heat cycling. When the sensor can't detect the flame properly, the control board assumes there's no flame present and shuts down the gas valve as a safety measure.

Your three-cycle pattern is programmed behavior - the control board gives the system three attempts before going into lockout mode. The 30-45 second fan pre-purge and 5-10 second flame detection window are normal timing parameters. Since you're getting ignition consistently, your igniter, gas valve, and control board are functioning correctly. The sail switch is also working since the fan is operating properly and ignition is occurring.

External factors like restricted airflow (which you've already checked with the floor vents), low propane pressure, or improper combustion air supply could theoretically cause similar symptoms, but given the consistent timing and the fact that ignition occurs reliably, the flame sensor remains the most likely culprit.

Before You Start

Locating your furnace access panel can be challenging depending on your RV layout. In most installations, the AFMD35 is accessible through an exterior compartment door, often on the roadside of the RV. Look for a rectangular door that's typically 12-16 inches wide. Some units have the access panel inside a larger storage compartment. Check your RV manual or look for Atwood service labels near potential access points.

Before beginning any work, disconnect the 12V power supply to the furnace, turn off the furnace at the thermostat, and shut off the propane supply at the tank or regulator. Allow the system to cool for at least 30 minutes if it's been running. Gather a 1/4-inch nut driver, needle-nose pliers, fine-grit sandpaper (220-400 grit), and a clean cloth. You'll also want a flashlight or headlamp since furnace compartments are typically dark.

Take a photo of the wire connections before disconnecting anything. The flame sensor on the AFMD35 is located near the burner assembly and looks like a thin metal rod extending into the combustion chamber. It's held in place by a single 1/4-inch hex screw and has one wire connection.

The Repair Process

Once you've accessed the furnace compartment, locate the flame sensor - it's the thin metal rod positioned to sit in the flame path when the burner fires. Disconnect the single wire connector (it should pull off easily) and remove the 1/4-inch mounting screw that secures the sensor bracket to the burner assembly.

Clean the sensor rod thoroughly using fine-grit sandpaper, working gently to remove any carbon buildup or corrosion. The rod should be shiny metal when you're finished. Avoid using steel wool or coarse abrasives that could damage the sensor surface. Wipe clean with a dry cloth and inspect the wire connection for corrosion or damage.

If cleaning doesn't resolve the issue, replacement is straightforward. The Atwood part number is 31058, available from RV parts suppliers for $35-45. Installation reverses the removal process - secure with the mounting screw, reconnect the wire, and ensure the sensor rod extends properly into the flame area. The tip should be positioned about 1/4 to 3/8 inch into the flame zone.

After reassembly, turn the propane back on, check all gas connections for leaks using soapy water, and test the system. The furnace should now complete its full cycle - fan pre-purge, ignition, flame establishment, and normal heating operation. If it continues the same shutdown pattern, double-check your sensor positioning and wire connection, or consider that the control board may have developed a fault.

Professional Help Needed

If replacing the flame sensor doesn't resolve the issue, the problem likely lies with the control board (part #31034, around $180-220) or there's an underlying gas supply issue that requires professional diagnosis. Control board replacement involves multiple wire connections and requires careful attention to proper sequencing during installation.

Call a certified RV technician if you're uncomfortable working with propane systems, can't locate the furnace access panel, or if the problem persists after flame sensor replacement. Gas appliance work requires specific knowledge of safety procedures and proper testing equipment. Most RV service centers can diagnose and repair Atwood furnace issues, with typical labor charges ranging from $120-180 for flame sensor replacement or $250-350 for control board replacement including diagnosis time.