RV Furnace Won't Ignite But Blower Runs: Complete Troubleshooting Guide

Quick Take

Your 2022 Thor Quantum SE28 is experiencing a classic furnace issue where an airflow fault (displayed as 'Air/Flow Limit Fault' on some control boards) is preventing proper operation. The furnace lighting for only 30 seconds before shutting down indicates the system is detecting insufficient airflow and triggering its safety shutdown sequence. Despite having adequate propane and a clean exterior/interior, the most likely culprits are a blocked exhaust vent on the outside of your RV, debris in the combustion air intake, or a failing sail switch inside the furnace unit. Your Suburban or Dometic furnace (standard in Thor units) has multiple safety sensors that will shut down operation if proper airflow isn't detected within the first minute of ignition.

This specific fault pattern - ignition followed by rapid shutdown with an airflow error - typically points to external obstructions rather than internal component failures. Since you confirmed propane flow to other appliances and stated everything appears clean, the issue is most likely in the external venting system or the sail switch mechanism that monitors airflow. The 30-second timeframe may indicate a safety timer issue, as most RV furnaces typically allow 7-10 seconds for ignition attempts, which gives the system a brief window to establish proper combustion airflow before triggering a protective shutdown.

Before calling for service, there are several diagnostic steps you can perform that will likely resolve this issue. Most Air/Flow Limit Faults on newer Thor units are resolved through proper vent cleaning and sail switch inspection or replacement, both of which are well within the capability of most RV owners with basic tools.

Common Causes

The Air/Flow Limit Fault in your Thor Quantum SE28 furnace system stems from the unit's built-in safety mechanisms designed to prevent dangerous operating conditions. The primary cause is almost always related to airflow restriction, but understanding the specific failure points will help you diagnose effectively.

External vent blockages represent the most frequent cause of this fault. Your furnace requires two separate air pathways: combustion air intake and exhaust output. Both vents are typically located on the exterior sidewall of your RV, often appearing as rectangular or round openings with protective screens or louvers. Even minor obstructions like spider webs, leaves, dust accumulation, or winter ice buildup can trigger the airflow sensor. The combustion air intake draws fresh air for the burning process, while the exhaust vent expels combustion gases. If either pathway becomes restricted, the internal pressure differential changes, causing the sail switch or pressure sensor to detect abnormal conditions.

The sail switch mechanism inside your furnace housing acts as the primary airflow monitor. This component consists of a lightweight metal or plastic flag that moves with airflow, triggering a microswitch when proper air movement is detected. Over time, dust, pet hair, or debris can accumulate around the sail switch, preventing it from moving freely or causing it to stick in the wrong position. Additionally, the microswitch itself can develop internal contacts that fail to make proper electrical connection, even when the sail switch moves correctly.

Blower motor issues can also manifest as airflow faults, though this is less common in newer units like your 2022 model. The blower creates the necessary air movement through the system, and if it's running at reduced speed due to worn bearings, accumulated debris on the fan blades, or electrical supply problems, insufficient airflow will trigger the safety shutdown. You can often hear if the blower is operating at full capacity by listening to the air movement when the furnace attempts to start.

Internal ducting problems, while less common, can create airflow restrictions that trigger the fault. Flexible ductwork can become kinked, crushed, or disconnected during travel or storage. Hard ductwork joints can separate or become blocked by objects that have fallen into return air grilles. Additionally, dirty or clogged return air filters throughout your RV can create enough back-pressure to affect the furnace's ability to move air properly through the system.

Preparation

Before beginning any furnace troubleshooting on your 2022 Thor Quantum SE28, ensure your safety and gather the necessary tools and materials. Turn off the furnace at the thermostat, shut off the propane supply at the tank or main valve, and allow 5-10 minutes for all components to cool down completely. Locate your RV's main electrical disconnect and propane shutoff valve in case emergency shutdown becomes necessary during your diagnostic work.

Essential tools for this repair include a Phillips head screwdriver, flat-blade screwdriver, flashlight or headlamp, shop vacuum with crevice attachment, and a mirror for inspecting tight spaces. You'll also need replacement parts potentially including a new air filter, furnace sail switch (part numbers vary by manufacturer but typically cost $25-45), and possibly new exterior vent screens. Having a multimeter available will help test electrical connections, though it's not always necessary for basic airflow troubleshooting.

Locate your furnace access panel, typically found inside a bedroom closet, under a dinette bench, or behind a removable panel in a hallway. Your Thor Quantum SE28 likely has either a Suburban SF-series or Dometic furnace, both of which have similar access requirements. Remove any items stored near the furnace area and ensure adequate lighting for your work space. Take photos of wire connections and component positions before disconnecting anything, as this will help during reassembly.

Safety preparation is critical when working with propane appliances. Ensure adequate ventilation in your work area and never use open flames or smoking materials nearby. Keep a Class B fire extinguisher within easy reach and make sure another person knows you're working on the furnace system. Test your propane leak detector or use soapy water to check for any gas leaks around connections before beginning work. If you smell propane at any point during the process, immediately shut off the propane supply and ventilate the area before continuing.

Repair Steps

Begin your repair process by accessing the external furnace vents on your 2022 Thor Quantum SE28. Walk around the exterior of your RV and locate the furnace intake and exhaust vents, typically positioned on the same side wall and often marked with small labels. The intake vent draws fresh air for combustion, while the exhaust vent expels burned gases. Remove any visible debris, leaves, or obstructions from both openings. Use a flashlight to inspect inside each vent opening as far as you can see, looking for spider webs, bird nests, or accumulated debris.

Clean both vents thoroughly using a shop vacuum with a crevice attachment to remove loose debris, followed by a damp cloth to wipe down the vent screens or louvers. Pay particular attention to the screen mesh, as even partial blockage can significantly impact airflow. If the screens are removable, take them out and wash them with soap and water, ensuring all soap residue is rinsed away before reinstallation. Check that both vents open and close freely if they have adjustable louvers, and lubricate the pivot points with a light spray lubricant if needed.

Move inside your RV and access the furnace compartment by removing the access panel. Before touching any components, take note of the furnace model number and serial number for future reference. Locate the blower assembly and sail switch, which will be positioned in the airflow path between the blower and heat exchanger. The sail switch appears as a small rectangular housing with a thin metal or plastic flag that should move freely when air passes over it.

Inspect the sail switch area carefully for any debris, dust accumulation, or obstructions that might prevent the sail switch flag from moving freely. Look for debris, dust accumulation, or physical damage. Gently move the sail switch flag by hand to ensure it pivots freely and returns to its rest position. Clean around the switch housing using compressed air or a soft brush, being careful not to bend or damage the delicate switch mechanism. If the sail switch appears damaged or doesn't move smoothly, replacement is typically straightforward and costs between $25-45 for the part.

Examine the blower motor and fan assembly for debris buildup or obstructions. Remove any visible accumulation of dust, pet hair, or other materials from the blower housing and fan blades. Check that the fan spins freely by gently rotating it by hand with the power off. Listen for unusual noises like grinding or squealing that might indicate bearing wear. Clean the blower housing thoroughly and ensure all air passages are clear.

Test the furnace operation by turning it on at the thermostat and observing the startup sequence. You should hear the blower motor start first, followed by the ignition sequence after a few seconds. Watch the sail switch during operation to verify it moves properly when airflow begins. If the furnace still shuts down after 30 seconds with an airflow fault, proceed to check internal ductwork connections and return air pathways.

Inspect all accessible ductwork connections inside the furnace compartment, looking for loose joints, disconnected ducts, or damaged flexible ductwork. Tighten any loose clamps and reconnect any separated duct sections. Check return air grilles throughout your RV for blockages or extremely dirty filters that could restrict airflow back to the furnace. Replace any dirty filters and remove any objects that might be blocking return air flow.

If airflow issues persist, test the furnace with the access panel removed to see if this improves operation. Sometimes the access panel itself can create airflow restrictions if not properly fitted or if the gasket material has deteriorated. With the panel off, attempt to start the furnace again while observing the sail switch and blower operation directly. This diagnostic step will help determine if the problem is related to the furnace compartment ventilation or internal component issues.

For electrical testing, use a multimeter to check voltage supply to the blower motor and sail switch circuits. With the furnace attempting to operate, you should see 12 volts DC at the blower motor connections and proper continuity through the sail switch when it's in the activated position. Poor electrical connections or low voltage can cause erratic operation that mimics airflow problems.

When to Get Help

Contact a qualified RV technician if your troubleshooting efforts don't resolve the Air/Flow Limit Fault after following the diagnostic steps above. Specifically, seek professional help if you discover damaged heat exchanger components, cracked or corroded exhaust piping, or if the furnace produces any unusual smells during operation attempts. These conditions can indicate serious safety hazards that require professional assessment and repair.

Professional service becomes necessary when electrical components beyond the sail switch require replacement or when the furnace control board shows signs of failure. Modern RV furnaces use sophisticated electronic controls that monitor multiple safety parameters, and control board failures can produce airflow fault codes even when airflow is adequate. Control board replacement typically costs $200-400 plus labor and requires proper programming for your specific furnace model.

If your diagnostic work reveals that the blower motor itself has failed or is operating at significantly reduced capacity, professional replacement is recommended. Blower motor replacement involves electrical work and proper component matching to ensure correct airflow volumes. Additionally, if you discover that the heat exchanger has cracks or corrosion damage, this represents a serious safety hazard requiring immediate professional attention and likely complete furnace replacement.

Schedule professional service if you're uncomfortable working with propane appliances or if local regulations require licensed technician work on gas appliances. Many states have specific requirements for propane appliance service, and warranty considerations on your 2022 Thor may require authorized service provider work. Professional diagnosis typically costs $100-150, while common repairs like sail switch replacement range from $150-250 including parts and labor. Complete furnace replacement, if necessary, typically runs $800-1500 depending on the unit size and installation complexity.

Keep detailed records of your troubleshooting efforts and any parts you've replaced, as this information will be valuable to service technicians and may be required for warranty claims. If the problem persists despite professional service, contact Thor directly as there may be known issues with specific furnace installations or manufacturing defects covered under your RV's warranty program.