Fan runs but no heat, furnace shuts off before warming up, or won't start at all — this guide covers the full ignition sequence, DSI board testing, sail switch, high-limit switch, and LP pressure diagnosis for Atwood and Suburban forced-air furnaces.
The RV furnace runs on propane but uses 12V for everything else — the blower motor, the ignition board (DSI board), the gas valve, and the safety switches. When the fan runs but there's no heat, you're looking at a failure somewhere in that 12V-controlled ignition chain, not a mechanical failure of the heat exchanger or blower itself.
Understanding the ignition sequence is the key to fast diagnosis. The furnace follows a strict order of operations every time it fires — and a failure at any point in that sequence produces a specific, identifiable symptom. This guide walks through each step with the tests that confirm or rule it out.
12V power is required for every test in this guide.
The furnace will not operate below approximately 10.5V. If your battery is weak or your converter is not charging, resolve the power issue first — a furnace that won't light due to low voltage looks identical to a board failure on the surface.
Specific test procedures, the exact readings you should see, and what it means when something's off. The kind of detail that tells you whether this is a $12 fix or a call to your dealer.
And you won't be working through it alone. A diagnostic chatbot built on 21 years of real field experience is coming soon — so when you hit a wall, you can ask the question directly.
Every Atwood and Suburban forced-air furnace follows the same sequence on a heat call. Knowing the order tells you exactly where to look based on what the furnace is doing when it fails.
| Step | What Happens | If This Step Fails |
|---|---|---|
| 1 | Thermostat calls for heat — 12V signal sent to DSI board | No response at all — check thermostat wiring and 12V at board |
| 2 | DSI board checks sail switch — airflow must be present | Blower runs but no ignition attempt — sail switch open or stuck |
| 3 | Blower starts — purge cycle (30–60 seconds) | Fan doesn't start — motor failure or capacitor (if equipped) |
| 4 | DSI board sends spark and opens gas valve simultaneously | Clicking with no gas smell — gas valve not opening |
| 5 | Burner ignites — flame sensor confirms flame within ~3 seconds | Flame lights then goes out — flame sensor dirty or failed |
| 6 | Furnace runs — high-limit switch monitors temperature | Furnace runs briefly then shuts off — high-limit tripping |
| 7 | Thermostat satisfied — gas valve closes, blower runs briefly to clear heat | Blower runs continuously — check thermostat set point vs room temp |
Low LP pressure is the most common furnace no-ignition cause that gets misdiagnosed as a board failure. The furnace gas valve requires adequate inlet pressure to open fully — at low pressure, the valve may partially open or not open at all, producing an ignition attempt with no flame or a weak flame that the sensor can't confirm.
The correct operating pressure at the appliance is 11 inches water column (WC) for propane. To test: connect a manometer to the test port on the furnace gas valve (most valves have a 1/8" NPT test port). With the furnace attempting to fire, read the inlet pressure. Below 10" WC indicates a supply or regulator problem. Correct pressure with no ignition confirms the gas valve or board is the issue.
The sail switch is a small flap inside the furnace blower housing. When the blower starts, airflow pushes the flap closed, completing a circuit that tells the DSI board airflow is present. If the sail switch doesn't close — because the flap is stuck, the spring is weak, or the blower isn't moving enough air — the board will not initiate ignition.
The high-limit switch is a thermal safety that opens the circuit if the furnace overheats. When it trips, the furnace shuts down mid-cycle — typically the blower keeps running but the gas valve closes and ignition stops. It resets automatically on most models once the furnace cools, which creates an intermittent complaint: furnace runs for 10–15 minutes, stops, then works again after a break.
The high-limit trips for a reason — don't simply bypass it. Common causes: blocked return air, blocked exhaust vent, undersized duct run, or a furnace in a compartment with insufficient combustion air. Locate and fix the cause before assuming the switch itself has failed. To test the switch: measure continuity across its terminals at room temperature. It should be closed (continuous). An open reading at room temperature means the switch has failed in the open position and needs replacement.
The DSI (Direct Spark Ignition) board controls the entire ignition sequence. It receives the thermostat call, monitors the sail switch, times the purge cycle, triggers the spark, opens the gas valve, and monitors the flame sensor. When the board fails, it usually produces one of three symptoms: no response to a heat call, repeated ignition attempts that never establish flame, or a board that sparks and opens the valve but can't confirm the flame.
| Test Point | Expected | If Wrong |
|---|---|---|
| 12V supply to board (red/black wires, board powered) | 11.5–13.8V DC | Check fuse on furnace circuit, check wiring from battery/converter |
| Thermostat signal wire at board (heat call active) | 10–13V DC | Check thermostat wiring, thermostat itself may be failed |
| 12V output to gas valve (during ignition attempt) | 11–13V DC | Board not sending signal — board likely failed |
| 12V at gas valve terminals (during ignition attempt) | 11–13V DC | Wiring break between board and valve |
| 12V at gas valve but valve doesn't open | Gas valve solenoid failure | Replace gas valve |
The flame sensor (sometimes called the flame rod or rectification rod) sits in the burner flame and uses a small AC current conducted through the flame to confirm ignition. A coated or corroded sensor can't conduct properly, so the board thinks there's no flame and shuts the gas valve — even when the burner is actually lit. This produces a flame that lights and immediately goes out (the gas valve closes within 3 seconds of ignition).
To clean the flame sensor: shut off LP at the tank, allow the furnace to cool completely, remove the sensor (single screw), and clean the rod with fine steel wool or 400-grit sandpaper. Remove all corrosion and coating from the rod tip. Reinstall and test. This fix resolves approximately 30% of "lights then immediately goes out" complaints without any part replacement.
The furnace exhaust vent and combustion air intake are typically combined in a two-pipe termination on the exterior of the coach. Mud dauber wasps build nests in these openings — it's the single most common cause of furnace failure at the start of camping season. The exhaust blockage causes incomplete combustion, the CO sensor trips, or the high-limit trips. The combustion air blockage starves the burner and causes repeated ignition failures.
Inspect the exterior vent termination before any other diagnosis if the furnace was stored for more than a few weeks. A small wire or compressed air clears a mud dauber nest in under a minute. Install a pest screen rated for furnace vents — standard window screen restricts airflow too much and will cause the same symptoms the pest was causing.