Quick answer: A forced-air RV furnace starts in a fixed order: thermostat call, blower, airflow switch, purge, spark, gas valve, flame sensing. Find where that order stops. No blower points to power. Blower but no flame points to propane supply, vents or a locked-out control. Lights then quits points to airflow or venting. A gas smell, a CO alarm, soot or erratic cycling is not a troubleshooting job.
This guide covers propane, forced-air furnaces such as Suburban and Dometic/Atwood Hydro Flame. The Truma Combi is a different heater and gets a short note. Read the manual for your exact model, because makers differ.
Stop and call a technician
This comes first on purpose. Stop and call a qualified RV technician or your propane supplier if any of these is true.
You smell propane. Dometic’s manual says to:
- Put out all open flames.
- Not operate electrical switches or a mobile phone, and not start the vehicle engine.
- Open windows and doors, and get everyone out.
- Close the propane cylinders from the outside.
- Call your gas supplier or the fire department.
- Have the whole gas system checked by qualified people before using it again.
A CO alarm sounds, or anyone has symptoms. The U.S. Consumer Product Safety Commission lists headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion. Its advice if you suspect CO poisoning: get outside to fresh air immediately, then call 911. Treat a sounding CO alarm the same way. Dometic notes a furnace can produce carbon monoxide, which has no odor.
Black soot at the outside vent. Suburban says incomplete combustion makes CO and soot, and a furnace showing black soot “should not be operated until the problem is corrected.” Dometic says shut it off right away and contact a technician.
It cycles erratically or is slow to light. Dometic says shut it down immediately and call a service agency.
It keeps locking out. Atwood says that after three tries with no ignition, shut down and contact your dealer, and do not keep cycling the thermostat.
Anything inside the furnace. Dometic says never to repair the furnace yourself and never to light the burner by hand. Burners, electrodes, sensors, boards, gas valves, regulators and gas pressure are technician work.
Detectors. Keep a working CO alarm and a working LP (propane) alarm. CPSC says to check CO alarm batteries regularly. We found no replacement interval in a source we could read, so check the date printed on each alarm. In winter, never let snow or objects block the furnace exhaust (Dometic).
How the furnace starts
Suburban’s service manual describes this order for its 12V electronic-ignition models:
- The thermostat calls for heat and the blower starts.
- At speed, airflow closes the sail switch, a safety that proves there is enough air for combustion.
- A purge of about 15 seconds clears the chamber.
- The gas valve opens and spark runs for about 7 seconds.
- The control looks for flame. After three failed tries it closes the valve and locks out.
- At the set temperature the valve closes and the blower runs on for about 90 seconds.
Timings vary by control board, and on some models the sail switch comes before the limit switch. Dometic says its control typically tries ignition three times. Atwood says to allow 30 seconds for the burner to light after the blower starts.
Diagnosis table: where does it stop?
| What you see or hear | Stage | Likely area (per manufacturers) | Owner or technician |
|---|---|---|---|
| No blower | Power or thermostat | Blown fuse, tripped furnace breaker, furnace in lockout (Dometic) | Owner: thermostat, power switch, fuse panel, battery. Technician: a tripped furnace breaker (Dometic) or a fuse that blows again |
| Blower runs, no spark | Airflow proof | Sail switch not closing: lint on blower wheel or sail switch, blocked return-air openings (Suburban) | Owner: check that return-air grilles are not blocked. Technician: cleaning inside |
| Blower and spark, no flame | Gas to burner | Air in LP line, thermostat not set high enough (Dometic); tank valve not fully open (Atwood) | Owner: tank valve and level. Technician: the rest |
| Lights, then locks out | Flame sensing | Electrode, wiring, module board (Suburban) | Technician only |
| Runs briefly, shuts off early | Limit or airflow | Blocked vents (Dometic) | Owner: clear outside grilles. Technician if it repeats |
| Smell, alarm, soot | Safety | Not a diagnosis | Stop |
Quick flow: should you keep going?
- Gas smell, CO alarm or black soot at the vent: stop, follow the safety steps above and call a pro.
- Blower does not start on a call for heat: check the thermostat, power switch, fuses and battery. Still nothing: call a pro.
- Blower runs but there is no flame: is the tank valve open and the tank not empty? If not, fix it and try at most 3 times. If yes, call a pro.
- It lights, then quits before the room warms: check the outside vents. If it repeats, call a pro.
- Otherwise it is working: book the yearly service.
If the blower never starts
Is the thermostat on and set above room temperature? Is the furnace power switch on? Dometic says gas will not flow with the switch off. It also lists a blown fuse, a tripped furnace breaker and a furnace in lockout.
These furnaces run on 12V. Suburban gives 10.5 to 13.5 volts DC and says the control waits in standby below 9.5 volts. Dometic’s codes include low voltage (8 to 9 V) and high voltage (16 to 17 V). Suburban warns never to power or test the furnace with a battery charger, which can exceed 14.5 volts and damage the board. A weak battery hurts other appliances too; see RV fridge not cooling.
To clear a Dometic lockout, turn the thermostat OFF for 10 seconds, then ON. Do not keep repeating it. A lockout that returns means call a technician.
If the blower runs but there is no flame
Check in this order.
- Propane. The tank valve must be fully open. Atwood says never to run it with the valve partly closed. If the water heater also fails, compare RV water heater not working.
- Air in the line. Dometic lists this as a cause. Its remedy is to run another appliance downstream, such as a cooktop, then reset the furnace, up to three times. Do this only with no gas odor.
- Thermostat. Dometic lists a thermostat not set to heat or not high enough.
- Vents. Suburban names screens, wasp nests and tape in the vent or intake as common causes of bad combustion.
Then stop. Spark with no flame, or a valve that will not open, is technician work. Suburban notes that gas pressure swings can come from a bad tank regulator, a kinked line or moisture.
If it lights, then shuts off
Suspect airflow first. Dometic says blocked vents can shut the furnace off before it reaches temperature. Suburban says lint on the blower wheel can make the motor drag and fail to close the sail switch. If vents are clear and it still quits, the cause is likely flame sensing: Suburban’s service hints point to electrode position, wiring and the module board. That is technician work, and it matches the “erratic cycling” warning above.
Suburban, Dometic, Atwood and Truma
Suburban diagnoses by symptom: no spark, spark but no gas, burner lights then goes out. The service manual we read (back cover dated 2007) has no LED code table. Park models run on 120V, others on 12V. Your model may differ.
Dometic DF series (DFSA, DFSD, DFMD, DFLD, DFLA) flashes an LED on the control board. DFLA can also use natural gas; the rest are propane. The table applies only to these models.
| LED | Fault | Lockout |
|---|---|---|
| Steady on | Internal circuit board failure | Hard |
| 1 flash | Limit switch or airflow problem | Soft |
| 2 flashes | Flame sense fault | Hard |
| 3 flashes | Ignition lockout | Soft (1 hour retry) |
| 4 flashes | High voltage, 16 to 17 V | Soft |
| 5 flashes | Low voltage, 8 to 9 V | Soft |
A soft lockout is timed and the control retries. A hard lockout needs the thermostat reset or the power switch turned off and on.
Atwood Hydro Flame AFM (AFMD16 to AFMD35, manual effective 8/15) lists the first four codes above. We did not read a manual for Atwood’s older DC-series furnaces, so use your own.
Truma Combi heats both space and water, so it is not a furnace. Truma’s tech tip says its manuals list error codes. Depending on the code, its steps include letting the unit cool, checking battery voltage, confirming the propane cylinder and shut-off valve are open, and looking for a blocked intake or exhaust outside. For some persistent codes, Truma points you to its Customer Care team. That page does not mention CO, so apply the CO rules above to any gas heater.
Yearly care
- Monthly: look at the vent for obstructions or soot (Dometic).
- Before heating season: Suburban says to clean the main burner and inspect the venting at least once a year. A furnace that has sat unused may need it more. Dometic says annual maintenance must be done by a qualified RV technician.
- Blower wheel and sail switch: lint can stop the sail switch from closing (Suburban). Ask the technician to clean them.
- Compartment: keep it closed while running, never use it for storage, and keep insulation away (Dometic).
Water stains near the furnace compartment are a separate problem; see RV water damage signs and repair. Check warranty terms before paying for a repair; see Is an RV extended warranty worth it?. We give no repair prices because we found no source we could verify.
FAQ
Why does the blower run but there is no heat?
The blower runs before ignition. If nothing lights, the stop is later: propane, air in the line, vents or a locked-out control.
Why is it blowing cold air?
Usually the burner is not lit or dropped out. Run the same checks. We found no single manufacturer-stated cause.
Is it safe to keep resetting the thermostat?
No. Atwood says to stop after three failed attempts and call.
Can I light the burner by hand?
No. Dometic says not to.
Does a furnace make carbon monoxide?
It can (Dometic, Suburban). Keep the vent and burner clean and keep a working CO alarm.
What do the flashing lights mean?
On Dometic DF models, see the table above. Confirm your model first.
Sources
Read 2026-10-20. Manuals change, so confirm against your own model’s manual. Claim-level detail is in the evidence file.
- Dometic, DF Series Furnaces Operating Manual (PDF created Sept. 2025).
- Suburban, RV Furnaces Service Manual, a copy hosted by a trailer dealer (official site: rvcomfort.com).
- Atwood, Hydro Flame AFM Series Technical Installation Manual, hosted by Forest River.
- Truma, Tech Tip: Truma Combi Error Codes.
- CPSC, Carbon Monoxide Information Center.
General information, not a substitute for your manual or a qualified technician. RVFixWise Editorial drafts with AI assistance and checks claims against the sources above.
