Complete repair guide for the Thor Palazzo – 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 the furnace failure
Start by turning off the furnace at the main power switch and allowing it to cool for 10 minutes. Use your digital multimeter set to DC voltage mode to test the control board terminals—you should read approximately 12V at the main power input. If you’re getting no voltage or erratic readings, the control board is likely faulty. Listen for clicking sounds when the thermostat calls for heat; a single click followed by silence typically indicates a failed igniter electrode, while no response at all points to a control board issue.
Step 2: Access and disconnect furnace components
Locate the furnace access panel on your Thor Palazzo (typically found in a basement cabinet or exterior compartment) and remove the cover by unbolting the four corner fasteners. Disconnect the propane supply line by turning the shut-off valve counterclockwise and waiting 30 seconds for residual pressure to bleed. Carefully disconnect the 12-wire harness from the control board by gently pulling the connector straight outward, noting the wire positions with a photo or sketch for reassembly.
Step 3: Remove and test the igniter electrode
Locate the igniter electrode—a small, ceramic-tipped rod positioned near the burner assembly. Unscrew the single mounting bracket bolt and carefully slide the electrode out of its housing, being careful not to touch the ceramic tip as oils from skin can cause ignition failure. Test the electrode resistance using your digital multimeter set to ohms mode; a functioning electrode reads between 40-90 ohms at room temperature. If the reading is outside this range or shows infinity (open circuit), replacement is required.
Step 4: Replace control board and sail switch
Remove the old control board by unbolting the two mounting brackets and sliding it free from the furnace frame. Install the replacement furnace circuit board by aligning the mounting holes and securing with bolts, leaving them finger-tight until final positioning. While the board is accessible, inspect and replace the furnace sail switch if the original shows corrosion or sticking; this switch must move freely to trigger the blower motor and should click audibly when manually deflected.
Step 5: Install the new igniter electrode assembly
Insert the new furnace igniter electrode into its mounting bracket, ensuring the ceramic tip aligns with the spark area approximately 1/8-inch from the burner ignition point. Tighten the mounting bolt to a snug fit (approximately 15-20 inch-pounds of torque) without over-tightening, as excessive pressure can crack the ceramic. Verify clearance by visually confirming no surrounding components will contact the electrode during furnace operation.
Step 6: Reconnect all electrical and propane systems
Reconnect the 12-wire harness to the control board terminals, matching each wire to its original position using your reference photo or sketch—incorrect polarity will prevent ignition and may damage the board. Reopen the propane supply valve by turning counterclockwise approximately 1.5 turns, then use soapy water to check all connection points for leaks; any bubble formation indicates a failed seal requiring reseating. Before closing the furnace panel, verify that the high-limit switch moves freely and isn’t corroded or blocked by debris.
Step 7: Test furnace operation and safety systems
Power on the furnace and set your thermostat to call for heat; you should hear the blower motor engage within 5 seconds, followed by a spark click at the igniter electrode and flame ignition within 10 seconds. Allow the furnace to run for 15 minutes and confirm hot air flows from vents and the sail switch actively cycles the blower. Finally, test your Propane/CO combo detector alarm by pressing its test button—it should emit a loud alarm within 3 seconds—and verify the furnace thermostat responds properly to temperature adjustments.