A furnace that will not light is usually not a dead furnace. Most failures trace to a sail switch that never closed, airflow blocked somewhere in the duct or return, a battery too flat to spin the blower to speed, or propane that never reached the burner. This guide works the way a technician works: it walks the ignition sequence in order, so your symptom tells you which step failed instead of sending you shopping for a control board you did not need.
Last reviewed: Sep 21, 2026, against current Suburban and Atwood service guidance. We update these guides when real-world data changes.
Nothing happens at all. No blower, no click. The furnace is not being asked to run, or it has no power. Check the thermostat is set to heat and above room temperature, then the furnace fuse, then the 12 volt supply. A tripped limit switch left open, or a hard lockout, also presents this way.
Blower runs, no heat, then shuts off. The sequence started and stopped before gas. This is airflow or propane, not ignition. Check battery voltage first, then the sail switch, then gas supply.
Lights, then shuts off after a few seconds. Gas and spark worked. The board did not confirm the flame. Suspect the flame sensor, the electrode, or the sensor's position in the flame.
Runs, but never gets warm. Usually ducting or return air. The furnace is cycling on its high limit because it cannot move enough air. Blocked returns, crushed ducts and sharp turns are the usual causes.
Works plugged in, fails on battery. Voltage. See the low voltage section below; this is the single most misdiagnosed furnace fault.
Soot on the vent, or a yellow flame. Stop. Do not run the furnace. That is incomplete combustion, and both Suburban and Furrion instruct owners to shut the unit down immediately.
Every furnace runs this sequence, and it never varies. The value of knowing it is that each step has a distinct symptom when it fails, so you can locate the fault instead of guessing. Timings below are Suburban's own published figures, with Atwood's alongside where they differ.
1. Power check. The control board verifies it has enough voltage. Suburban boards look for at least 9.5 volts and go into standby below that, resuming when adequate power returns. Nothing else happens until this passes.
2. Thermostat calls for heat. The board confirms the sail switch circuit is open before starting, which is a self-check. Then it energizes the blower. On Atwood units there is a relay delay of 1 to 25 seconds here, or 1 to 2 seconds if the relay is on the board itself.
3. Airflow is proven. The blower must come up to speed and push the sail switch closed. Suburban's older time-delay models want roughly 75 percent of normal rpm at 12 volts before ignition is allowed. If the sail switch circuit is still open 30 seconds after the blower starts, the board locks out and stops the blower.
4. Purge. Suburban runs a pre-purge of about 15 seconds; Atwood's is 15 to 17 seconds. This clears the combustion chamber before gas is introduced. It is why a furnace makes you wait after the blower starts.
5. Gas and spark. The board opens the gas valve and fires the electrode. Suburban allows 7 seconds of ignition time; Atwood about 6. Older Suburban time-delay models wait 12 to 18 seconds after the purge before opening the valve.
6. Flame is confirmed. The board must sense the flame or it shuts the gas off. Suburban's published figure is at least 7 microamps within 7 seconds of ignition, measured in series with the flame sensor.
7. Retries, then lockout. Three failed attempts and the board gives up. Suburban runs the blower for 3 minutes and then shuts down in lockout. Atwood enters a soft lockout that times out after one hour, or a hard lockout that needs the thermostat cycled.
8. Post-purge. When the thermostat is satisfied, the gas valve closes and the blower keeps running for 90 seconds to clear the chamber. A furnace that runs on for a minute and a half after the heat stops is behaving correctly.
Why this ordering matters: a board that never reaches step 3 cannot have a flame sensor problem, and a furnace that reaches step 6 has already proven its sail switch and its gas supply. Reading the sequence backwards eliminates most of the search.
The sail switch is a paddle sitting in the blower's airstream. When enough air moves, it closes and completes a circuit that tells the board it is safe to add gas. Suburban describes it as a safety device that will not let ignition occur until it sees 75 percent of the motor's rpm, and Atwood calls it a safety device that ensures airflow before ignition. It is the furnace's way of proving the chamber is being ventilated.
Why it fails. Debris between the paddle and the contacts, a bent paddle, loose connections, or anything that restricts airflow. A weak blower from low voltage will also stop it closing, which is why voltage comes first.
The trap that wastes the most time. The control board cannot test the sail switch by itself. It only sees the whole safety circuit, so a genuinely bad switch and a blocked duct are indistinguishable to it. If airflow is restricted anywhere in the system, the board reports what looks exactly like a sail switch failure. Service technicians put it plainly: the furnace circuit board is not intelligent, it can only check the safety circuit the sail switch belongs to. So before replacing the switch, confirm the ducts and the return air are clear.
How to test it. With the blower running, you want to see the switch pass voltage. If one side shows 12 to 13 volts and the other side shows little or nothing, the switch is not closing. A bench continuity test also works: an open switch reads OL, a closed one reads near zero ohms.
Where it lives. On Suburban direct-vent furnaces the sail switch sits on the blower intake side, mounted to the blower housing between the motor and the return-air opening. On Atwood units it sits between the valve and the ignition control, just above the limit switch. One caution from Atwood's own manual: the limit and sail switches are reversed on 79 and 80 series units.
One thing not to try. Do not jumper the sail switch to make the furnace run. The controller sees the jumper and will not start the blower. These switches exist to keep gas out of an unventilated chamber.
The furnace is the heaviest 12 volt load in the RV, and it is fussy about voltage in a way your lights are not. Suburban publishes a furnace voltage range of 12 volts DC nominal, minimum 10.5 and maximum 13.5, and its module board checks for a minimum 9.5 volts, going into standby below that. The older time-delay manuals are explicit: low supply voltage will not provide sufficient motor rpm to engage the sail switch.
This is why a furnace so often "works on shore power but not on battery." The battery may happily run lights and a water pump while being too weak to spin the blower to the speed the sail switch needs. Charge the battery, or plug in, before you replace any part.
Truma publishes the same idea as numeric thresholds: a warning at under 10.4 volts, an undervoltage fault at under 10.0 volts, and an overvoltage fault above 16.4 volts.
One warning from Suburban's own manual: never use a battery charger to power or test the furnace. Chargers can deliver more than 14.5 volts and damage the module board.
Because the furnace is the highest-demand gas appliance on board, it is the first thing to fail when gas supply is marginal. A stove burner may light happily while the furnace locks out.
Line pressure. Suburban specifies a minimum of 11 inches of water column and a maximum of 14.
The pressure drop test. This is the most useful furnace-specific gas check and the one most owners never run. Measure line pressure, then cycle the furnace and watch the pressure on demand. Suburban's instruction is that the drop should not exceed half an inch of water column. A larger drop indicates a faulty regulator, a restriction or kink in the gas line, or moisture contamination.
Moisture and cold. Suburban notes that lockout can occur if gas pressure fluctuates when the thermostat calls for heat, caused by a failing regulator, an obstruction or kink, or moisture in the regulator or lines. Truma is explicit about cold weather: butane concentration in the cylinder is too high for heating below roughly 10 degrees Celsius, and a frozen regulator needs a heater rather than more spark.
The excess flow device. If the tank valve is opened too quickly, the excess flow valve in the pigtail can trip. The resulting trickle is enough for a pilot light and nowhere near enough to sustain a furnace. Close the valve, wait, and reopen slowly.
The limit switch is an in-line safety that opens at a set temperature and cuts power to the ignition system, protecting the furnace from overheating. When it trips, the furnace stops heating. It is doing its job, and the cause is usually airflow, not the switch.
Suburban's own guidance names the causes: insufficient return air will cause the furnace to overheat and cycle on limit, a minimum duct area must be maintained through the whole system including the register, and sharp turns in the duct raise static pressure and cause limit cycling. Atwood's consumer manual adds that air boosters should never be installed, because they cause limit cycling and erratic sail switch behavior.
This is a safety issue, not just a comfort one. A federal recall on Forest River and Coachmen units describes the mechanism exactly: a furnace short cycling from an inadequate cold air return can overheat and puncture the burn chamber, allowing carbon monoxide into the coach. The remedy was new return air and ducting.
A caution about the limit switch. Many limit switches are automatic reset, and some Suburban units use a manual reset plunger. But "reset it and keep going" is treating the symptom. If a limit switch trips repeatedly, the documented cause is airflow, and the fix is the duct or the return, not the switch.
When the furnace lights and then shuts off seconds later, gas and spark are working and the board is not seeing the flame. Suburban senses the flame through the spark wire and electrode rather than a separate probe on some models, which is why electrode position matters so much on those units.
Suburban's own alignment specs, worth having in front of you: electrode-to-ground gap no more than 1/8 inch, the gap between sensor and ground should be twice the electrode gap, the electrode sits about 1/4 inch above and directly over the burner slots, and the sensor probe should sit in the inner blue cone of the flame, about 1/4 to 5/16 inch above the burner.
The order to check. Confirm the sensor is over the burner slots. Check the sensor wire connections. Check wire continuity. Then measure microamps. If the reading is below 7, Suburban says to adjust the sensor position and check for carbon deposits on the probe.
Atwood's manual states the principle without publishing a number: as long as the board receives the minimum microamps from the electrode assembly, it will not lock out. We could not find a published microamp figure for Atwood, and we are not going to invent one.
A Suburban warning worth repeating: never run the furnace with the electrode wire disconnected or the assembly removed, and make sure the spark never reaches the flame sensor portion. Sparking to the sensor will damage the module board.
This is where brands differ most, and where bad information is most common.
Atwood and Dometic publish a real table, from the manufacturer's own service manual. One flash with a 3 second pause is an airflow or limit fault. Two flashes is a flame sense fault. Three flashes is an ignition lockout fault. A steady light with no flashing is an internal control failure. The consumer manual adds the lockout type: a soft lockout is a timed condition that will make further attempts, while a hard lockout requires resetting the thermostat or cycling the power switch. On Atwood's two-stage Excalibur units the codes are numeric: 1 low input voltage, 2 ignition failure, 3 open high limit, 4 stuck APS which is the sail switch, 5 module fault.
Suburban does not publish a standardized blink code table, and this is worth saying plainly because so many websites claim otherwise. Suburban's service manual provides troubleshooting flow charts and service hints, not an LED code chart. Its owner material describes lockout behavior without codes, noting that after lockout the blower runs for 5 minutes and then shuts off, and the system must be reset at the thermostat. Several third-party sites publish Suburban blink code tables, and they contradict each other, which is a reliable sign they were generated rather than sourced. Use your model's flow chart, and the label on your unit, instead.
Truma publishes the most complete codes. On VarioHeat, E2H or E16H means flame not detected, most often a closed gas valve or blocked combustion air or exhaust. W27H is a blocked warm air outlet or circulated air intake. W29H is imminent undervoltage. On Combi, 17 is hot water mode with an empty container, 18 is a blocked air outlet, 41 is blocked electronics, 43 is excess voltage, 44 is undervoltage, and 255 is a missing 12 volt supply or a lost connection to the control panel.
Furrion is the odd one. Its manual states that error codes can only be retrieved by a qualified RV service provider with a Furrion diagnostic PCB, so there is no owner-readable code on those units. It does publish a reset procedure: thermostat off, wait 60 seconds, on to heat for more than 20 seconds, off, then on again within 20 seconds. Or remove power for about 5 minutes. Furrion also says that if the furnace still does not ignite after three attempts, contact a qualified service provider.
Mud daubers and other insects nesting in the intake, the burner tube or the heat exchanger is one of the most commonly reported furnace blockages in the field. We looked for it in Suburban, Atwood, Truma, Furrion and the RV standards and could not find a manufacturer service document that names insects as a failure mode, so we are attributing it where it belongs: to technician and owner experience, not to the manufacturer's list.
The mechanism is the one the manufacturers do document. A blocked combustion air intake or exhaust obstructs airflow, combustion becomes incomplete, and incomplete combustion produces soot. Suburban's own words: soot is formed whenever combustion is incomplete, this is your visual warning that the furnace is operating in an unsafe manner, and if soot is present, shut it down immediately and contact a qualified service person. Furrion says the same and adds that owners should inspect the vent monthly during heating season.
One case reported on an owner forum describes a nest in the heat exchanger blocking hot gases until the tube failed, letting combustion gases mix with the heated air. That is the mechanism to take seriously.
A furnace is a combustion appliance inside the space you sleep in. This section is short on purpose.
The CDC describes carbon monoxide as an odorless, colorless gas that kills without warning, and lists headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion as the common symptoms, often described as flu-like. People who are sleeping can die before they have symptoms. The CDC also notes that more than 400 Americans die each year from unintentional carbon monoxide poisoning not linked to fires.
Stop and evacuate if you smell gas, if the flame is yellow or lazy rather than steady blue, if you see soot at the vent, or if the alarm sounds. Furrion's gas-smell protocol is specific: evacuate everyone, shut off the gas at the container, do not touch electrical switches or use a phone inside, do not start the engine or generator, and contact a gas supplier or qualified technician.
A cracked heat exchanger is the end of the road. Two federal recalls describe the failure. One covers units where a short-cycling furnace overheated and punctured the burn chamber, allowing carbon monoxide poisoning; the other covers units shipped with the furnace exhaust vent missing from the exterior wall. Neither is a repair an owner should attempt, and a punctured heat exchanger means furnace replacement rather than a part.
Standard attention is worth noting: the 2026 edition of NFPA 1192, the RV standard, added carbon monoxide requirements, and it took effect September 1, 2026. Older RVs may predate those provisions.
The manufacturer language here is unusually blunt, and it is worth quoting rather than paraphrasing. Furrion: never attempt to repair the furnace yourself. Atwood: never attempt to repair the furnace yourself, seek the help of a qualified service person. Truma: only a qualified service technician may clean and maintain the unit, and after any misfire a qualified technician must inspect it.
Reasonable owner work: visual inspection, clearing vents and return air, removing debris from the exterior intake, cleaning the flame sensor and electrode with power and gas off, checking and replacing a sail switch or limit switch, and verifying voltage and gas pressure with a meter or manometer. The checks in this guide are all in that territory.
Stop and hand off: anything involving gas piping or the gas valve, the sealed combustion chamber, the heat exchanger, and board-level diagnosis once airflow and the safeties have been ruled out. Suburban is direct about the board itself, which is not field repairable and should be replaced rather than repaired, because attempting a repair may alter it and cause unsatisfactory operation.
Follow the sequence, not the parts catalog. Check voltage and airflow before anything else, because low voltage and blocked ducts mimic every other fault in this guide. If your symptom is "blower runs, no heat," you are looking at a sail switch, airflow or gas problem, and there is no reason to price a control board yet.
We could not find manufacturer or government published repair pricing, so these come from RV service companies and are ranges, not quotes. Shop labor rates and access vary enormously, and a furnace buried behind a cabinet takes longer than one with an exterior hatch.
Sail switch or limit switch: roughly $150 to $325 installed, with the part itself often under $20.
Ignitor or control board: roughly $275 to $700 installed. The board is the expensive part in this group.
Blower motor: roughly $225 to $600 installed.
Full furnace replacement: roughly $900 to $2,600 installed, which is also the answer when the heat exchanger is cracked. One published labor rate we found was $260 per hour, which is a useful number to hold when you compare an estimate against the price of a new unit.
The blower runs first because the furnace proves airflow before it allows any gas. If the flame never starts, the sequence stopped somewhere between the blower starting and the burner lighting. The two usual suspects are the sail switch not closing (often because the blower is not reaching enough speed, or the paddle is stuck) and propane not reaching the burner. Check battery voltage first, then the sail switch, then gas supply.
The burner lit, so gas and ignition are working. It shut off because the control board did not confirm the flame. On Suburban units the board expects to sense at least seven microamps within seven seconds. Common causes are a dirty or mispositioned flame sensor, a cracked or carboned electrode, or a sensor not sitting in the flame. Cleaning and repositioning the sensor is the first fix.
The board cannot test the sail switch on its own, it only sees the whole safety circuit, so a bad sail switch and a blocked duct look identical to it. Test the switch directly: with the blower running you should see full battery voltage across the switch when it closes. No voltage on the outgoing side while the incoming side has 12 volts means the switch is not closing. Debris between the paddle and the contacts is the most common cause.
Because the furnace is the biggest 12 volt load in the RV and it refuses to run below a voltage floor. Suburban module boards check for a minimum of about 9.5 volts and go into standby below that. A tired battery can run lights and a water pump happily and still fail to spin the furnace blower fast enough to close the sail switch. Charge the battery or plug in before you replace any parts.
It depends on the brand, and the difference matters. Atwood and Dometic units publish a real code table: one flash is an airflow or limit fault, two flashes is a flame sense fault, three flashes is an ignition lockout, and a steady light with no flashing is an internal control failure. Suburban does not publish a standardized blink code table in its own service manuals, so treat third party Suburban code charts with suspicion and use your model's flow chart instead.
Stop at the sealed combustion chamber, the heat exchanger, the gas valve, and any gas piping. Manufacturer manuals are blunt about this and tell owners never to repair the furnace themselves. Soot on the vent, a yellow or lazy flame, or any smell of gas are immediate stops: shut the furnace down and have it serviced. A cracked heat exchanger can put carbon monoxide into the living space.