RV furnace not working: The troubleshooting guide

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. Check it before you price any part, and see the low voltage section below.

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 forced-draft RV furnace runs this same sequence in this same order. The thresholds and the timings differ between makers and between control board generations, so match every figure to the board in your own unit. The value of knowing the order is that each step produces its own symptom when it fails, which narrows the possibilities instead of widening them.

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. Before you probe anything on the board, take the 12 volts out of it. Pull the fuse or switch the disconnect off, and treat every connector as live until a meter proves otherwise. A probe that slips off a control-board pin and touches the chassis is a dead short through the board, and on a live circuit it can weld the probe tip and burn you.

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, and on those furnaces a sail switch that never closes leaves the blower running until the thermostat is satisfied or switched off, because there is no board in charge of the motor. The fan control module boards do own the motor: 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. On older Suburban time-delay models the purge itself runs 12 to 18 seconds, and the valve opens at the end of it.

6. Flame is confirmed

The board must sense the flame or it shuts the gas off. Suburban's service literature puts its own figure at least seven microamps within seven seconds of ignition, measured in series with the flame sensor. The control makers publish a much lower minimum for the modules themselves: Fenwal's direct spark controls specify 0.7 microamps as the least current that keeps them from locking out. So a burner the board keeps lit is a burner the board is sensing, whatever your meter reads against Suburban's higher number.

7. Retries, then lockout

Three failed attempts and the board gives up. On Suburban's fan control module board, a failed ignition de-energises the gas valve at once and the blower keeps running for 3 minutes before the board shuts down in lockout; the 5-minute figure belongs to a different condition, where the limit switch circuit stays open that long. Atwood's board locks out for one hour after three tries, and its hard lockout needs the thermostat cycled or the power removed.

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.

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's wording is that the room air blower must be running at approximately 75 percent of its normal rpm before ignition can occur, and Atwood's is that the sail switch is a safety device that insures air flow before ignition. Both phrases describe the same job: proving the chamber is being ventilated before gas arrives.

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. 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 furnaces the sail switch sits in the blower's airstream, where the room air wheel pushes its paddle closed. On Atwood units it sits between the gas valve and the ignition control, just above the limit switch. One note from Atwood's own manual: the limit and sail switches are reversed on 79 and 80 series units, and Suburban's diagrams show the order varying by model too.

One thing not to try

Do not jumper the sail switch to make the furnace run. Its whole job is to keep gas out of a chamber the blower has not cleared, and defeating it is how a furnace lights when it should not.

The furnace is a heavy 12-volt load, and it is strict about voltage in a way your lights are not. Atwood's manual puts its 79 series furnace at 3.4 amps while it runs. 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.

If the supply voltage is the problem rather than the furnace, work the 12-volt path in order: battery terminal voltage under load, the master disconnect, the main DC fuses, the converter output, and the chassis ground returns. The guide to finding 12-volt faults walks that path, with a figure for each point and a method for finding where the volts are being lost.

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 a numeric threshold: its imminent-undervoltage warning appears when the battery falls below 10.4 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 lists a butane-heavy cylinder among the causes of its flame-not-detected fault, which is why the same furnace can work at home and struggle in the cold.

The excess flow device

The pigtail fitting contains an excess flow valve, and those valves are surge sensitive: they close on a sudden rush of flow rather than on a steady one, which is what a tank valve opened quickly into empty lines produces. Treat the tank shutoff valve as either fully open or fully closed rather than throttled part way, and open it slowly rather than snapping it. If the furnace locks out straight after a refill or a tank change, close the valve, wait, then open it slowly before hunting for a furnace fault.

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.

A safety issue, not a comfort one

NHTSA recall 24V047, 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 reset themselves, and some have a manual reset plunger you have to press. 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: the spark gap between the electrode and ground is 1/8 inch, the electrode tip sits 3/16 inch from the back of the burner lance and over the burner ports, and on the remote flame sense arrangement the gap from the ground probe to the sensor runs at about 1/4 inch, which is twice the spark gap. Suburban draws local sense and remote sense separately, and gives the dimensions per drawing.

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 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. Suburban is the one that puts a figure on it, at seven microamps, against the 0.7 microamps the control makers specify as their own minimum.

Suburban's warning states: 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.

Diagnostic codes and diagnostic ability differ widely between makers and between control boards.

Atwood, now owned by Dometic, publishes a real code chart in its own service manuals, and which chart applies depends on the board. On the 79 series ignition board the five conditions are counted in flashes: one low input voltage, two ignition failure, three open high limit, four stuck sail switch, five module fault. The same chart calls the first three soft lockouts, which are timed and let the board try again, and the last two hard lockouts, which need the thermostat or the power cycled. Atwood's consumer material describes a different set for other boards, where one flash is an airflow or limit fault, two is a flame sense fault, three is an ignition lockout, and a steady light with no flashing is an internal control failure.

Suburban publishes no standardized blink code table, and its service manuals give troubleshooting flow charts and service hints rather than an LED code chart. Its lockout behaviour is described without codes, and it depends on the board. On the older time-delay furnaces the unit stays in lockout with the blower running until the thermostat is turned off, and the sequence can be started again once the blower has stopped. On the fan control module boards a failed ignition de-energises the gas valve and the blower runs for 3 minutes before the board shuts it down. Third-party Suburban blink code tables contradict each other. Use your model's flow chart, and the label on your unit, instead.

Truma publishes the fullest set of 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, and this one is in the manufacturers' own service literature rather than only in the field. Suburban's NT series service manual lists it directly, describing how insects such as mud daubers and wasps build nests inside the combustion air housing after coming in through the fresh air intake, and instructing that the housing be cleared and the combustion air wheel checked for warpage. Atwood's furnace service manual gives the same instruction in its own words, telling technicians to check the air intake and flue areas for internal obstructions such as wasp or bird nests.

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.

A nest that blocks the combustion air housing is the blockage the manuals warn about, and the failure it can lead to is worse than a cold night: a restriction that raises the temperature until the heat exchanger tube fails, after which combustion gases mix with the air you breathe.

A furnace is a combustion appliance inside the space you sleep in.

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 NHTSA recalls describe the failure. 24V047 covers units where a short-cycling furnace overheated and punctured the burn chamber, allowing carbon monoxide poisoning; 25V528 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.

NFPA 1192 is the standard new RVs are built to, and it has been revised since yours was built, so the provisions your furnace was designed against may not be the current ones. Older RVs may predate provisions that are in the standard now.

The makers are direct about the limit. 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.

What a repair costs comes down to two things: the part, and how hard the furnace is to reach. A sail switch, a limit switch or an ignitor is a cheap part. A control board or a blower motor is not, which is why confirming the fault before authorising either of them matters. Labour is charged by the hour, and access is the swing factor, so a furnace behind a cabinet or under a bed is a longer job than one with an exterior hatch. A cracked heat exchanger is not a part at all: the furnace gets replaced.