Complete repair guide for the Thor Gemini – Furnace Igniter & Control Board Replacement. Follow these steps to diagnose and fix the issue yourself.
Parts & Tools You’ll Need
- Suburban RP-35Q 35,000 BTU/h RV Replacement Core for Suburban Furnace Series SF-35, SF-35Q, SF-42, SF-42Q, and SF-Q (2609A) — Suburban/Atwood RV furnace (replacement unit)
- Fit For Suburban RV Furnace Parts 232286,Single Probe Gas Furnace Igniters Electrode with Wire Assembly, Camper Furnace For Suburban 232286 Above 934701426 SF-20, SF-25, SF-30, SF-35 (SF Series) — Furnace igniter electrode
- DTAIR 33082 Sail Switch Replacement for Select Dometic Atwood RV Furnace(Pack of 2) — Furnace sail switch
- 520814 Rv Water Heater Module Board Ignition Control Circuit Board Compatible with Suburban Furnace SW4D, SW6D, SW6DE, SW12D, SW6DEM RV Water Heaters,Replace 520814 520820 520871 33550L (With lid) — Furnace circuit board / control board
- DTAIR 33082 Sail Switch Replacement for Select Dometic Atwood RV Furnace(Pack of 2) — Furnace high-limit switch
- Suburban 232684 RV Furnace 12v SF-Series DC Blower Motor, SF-35, SF-35F, SVF-35, SF-42, SF-42F OEM Caliber — Furnace blower motor (12V DC)
- RV Carbon Monoxide & Propane Gas Alarm, Briidea Dual LP/CO Detector with Separate LED Indicator Light, 100dB Loud Alarm, 12 VDC, Black — Propane/CO combo detector alarm
- FKM Pro Digital Multimeter Tester TRMS 6000 Counts,Smart Rechargeable Voltmeter 5″Color LCD,Auto-Ranging Automotive Multimeters,for AC/DC Current/Voltage,NCV,Ohm,Capacitance,Resistance,Continuity,Temp — Digital multimeter
Step 1: Diagnose Furnace Failure & Prepare Workspace
Start by listening for the furnace’s ignition click when the thermostat calls for heat—if you hear nothing or repeated clicking without ignition, your igniter electrode or control board is likely faulty. Turn off propane at the tank and disconnect the 12V DC power supply to the furnace breaker, then allow the unit to cool for 15 minutes. Gather your digital multimeter, replacement igniter electrode, control board, and sail switch, and clear your work area to ensure safe access to the furnace compartment.
Step 2: Remove Furnace Cover & Access Components
Unbolt the furnace access cover (typically 4 bolts on the Thor Gemini model) and set it aside, exposing the burner assembly and control board mounted on the sidewall. Take a photo of the wire harness connections with your phone before disconnecting anything—you’ll reference this during reassembly. Gently disconnect the white, red, and black igniter wires from the control board by carefully pulling the spade connectors straight back without twisting.
Step 3: Test Igniter Electrode With Digital Multimeter
Set your digital multimeter to the resistance (ohms) setting and probe the igniter electrode terminals to check continuity—a healthy electrode reads between 2-4 ohms. If your reading is above 5 ohms or shows infinite resistance (open circuit), the electrode has failed and must be replaced. Note that a cracked or corroded electrode will not generate the 10,000+ volt spark needed for ignition, even if it shows resistance.
Step 4: Remove & Replace Furnace Igniter Electrode
Unbolt the igniter electrode bracket from the burner assembly (typically one 1/4-inch bolt) and slide the electrode out toward you, noting its exact position relative to the flame rod. Install the new furnace igniter electrode into the same position, ensuring the ceramic insulator doesn’t contact any metal parts, and tighten the mounting bolt to 8-12 inch-pounds—snug but not over-tightened, as excessive force cracks the ceramic. Reconnect the igniter wires to the new electrode, matching your photo reference for correct polarity.
Step 5: Inspect & Replace Furnace Control Board
Examine the control board for visible burn marks, cracked capacitors, or corroded solder joints—any of these indicate failure. Unbolt the control board from the furnace sidewall (typically 2 bolts) and carefully disconnect all remaining wire harnesses, including the sail switch and high-limit switch connections. Install the replacement circuit board in the identical orientation, reconnect all wires using your reference photo, and ensure the board sits flush against the mounting surface before tightening bolts to finger-tight plus a quarter turn.
Step 6: Verify Sail Switch & Test Airflow Circuit
Check that the furnace blower motor spins freely by hand and that the sail switch (air flow sensor) moves smoothly when you gently push it—these components must function for the control board to complete its safety circuit. With power still disconnected, inspect the sail switch wires for corrosion and clean any lint from the blower housing using a soft brush. Reconnect the sail switch to the new control board, ensuring the connector seats fully with an audible click.
Step 7: Restore Power & Execute Final Safety Test
Reconnect the 12V DC power supply to the furnace breaker and turn propane on at the tank, then set your thermostat 5 degrees above the current room temperature to trigger ignition. Listen for the igniter click followed by a whoosh of ignition within 10 seconds—if ignition fails, turn off propane immediately and recheck all wire connections against your reference photo. Once ignition succeeds, verify the furnace runs for 3-5 minutes without shutting down prematurely, confirm the blower motor activates within 30 seconds, and test your propane/CO combo detector to ensure it reads 0 ppm before declaring the repair complete.