Complete repair guide for the Tiffin Allegro Bus – 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 Ignition Failure
Start by disconnecting the negative battery terminal to isolate all power, then locate your Tiffin Allegro’s furnace unit typically found in the basement compartment near the propane tank. Listen for a clicking sound when you turn on the furnace—if you hear rapid clicking but no ignition flame appears within 10 seconds, your igniter electrode is likely fouled or the control board has failed. Use your digital multimeter set to DC voltage to test for 12V at the furnace connector; if voltage is present but the igniter doesn’t glow red, the electrode needs replacement.
Step 2: Disconnect Power and Propane Supply
Reconnect the battery, then locate and turn off the propane supply valve at your main tank, waiting 2-3 minutes for residual gas to clear from the line. Switch off the furnace and all house power at the main breaker panel, then disconnect the negative battery terminal again to completely de-energize the system before opening any furnace compartments. This dual isolation method prevents accidental ignition and electric shock hazards during component replacement.
Step 3: Remove Old Igniter Electrode Assembly
Open the furnace access panel and locate the igniter electrode—a ceramic-tipped probe about 2 inches long mounted near the burner assembly with a single bracket screw. Disconnect the wire connector from the back of the electrode (note the terminal position for reinstallation), then remove the bracket screw using a 7/16-inch wrench. Carefully slide the electrode out of its mounting bracket, inspecting the ceramic tip for cracks, discoloration, or carbon buildup; black crusty deposits confirm ignition failure due to fouling.
Step 4: Install New Igniter Electrode and Connector
Insert the new furnace igniter electrode into the mounting bracket with the ceramic tip pointing toward the burner flame path, then secure with the bracket screw torqued to 20-25 inch-pounds (firm but not over-tightened). Reconnect the wire terminal to the back of the electrode matching your noted position, ensuring a click-fit connection with no exposed copper. Check that the electrode tip is positioned approximately 3/8-inch away from the burner orifice for proper spark-to-fuel ignition geometry.
Step 5: Replace Furnace Control Board and Connections
Locate the circuit board mounted inside the furnace cabinet (usually a 6×4-inch board with multiple wire terminals), and photograph the wire positions before disconnecting anything. Disconnect all colored wires from the old control board noting their terminal positions, then remove the board’s mounting bracket (typically 2-3 Phillips screws). Install the new furnace circuit board in the same position and orientation, reconnecting each wire terminal precisely as documented—incorrect wiring will prevent ignition or cause safety switch lockouts.
Step 6: Verify All Safety Components and Seals
Before closing the furnace compartment, inspect the sail switch (a small spring-loaded paddle near the blower intake) for free movement and proper positioning, as this safety device prevents fuel flow without adequate air flow. Check that the high-limit switch (usually a round or rectangular reset button) is accessible and not in the tripped position; press to reset if needed. Ensure all access panels and compartment seals are properly reinstalled with gaskets intact, as air leaks will prevent proper furnace operation and trigger safety shutdowns.
Step 7: Test Furnace Operation and Safety Systems
Restore all power by reconnecting the battery and main breaker, then open the propane supply valve and wait 30 seconds for pressure to stabilize. Turn on the furnace thermostat to heat mode and listen for the igniter clicking sound followed by flame ignition within 10 seconds; the burner should run for 15-20 seconds then establish steady flame. Verify that your propane/CO combo detector alarm does not trigger (indicating no gas leaks), and test the furnace cycling by raising and lowering the thermostat to confirm automatic shutoff works properly—allow 2-3 complete cycles to confirm reliability before resuming normal use.