RV Furnace Won't Ignite: Troubleshooting Sequence
An RV furnace that blows cold air, or clicks and never lights, fails in a predictable chain, and the fastest diagnosis comes from understanding that chain. Suburban and Dometic (Atwood) furnaces both follow the same sequence: thermostat calls for heat, blower starts, a sail switch proves the blower is moving air, the control board waits through a purge delay, the igniter sparks, the gas valve opens, and a flame sensor confirms ignition. When the furnace fails, it fails at one specific link. Your job is to figure out which one. One safety note before starting: if you ever smell propane at the furnace, shut off the gas and stop. That is professional territory.
Symptom 1: Nothing Happens at All
Thermostat up, and silence. This is a 12V power problem, not a furnace problem. Check battery voltage first; furnaces need healthy voltage to run the blower, and below roughly 10.5V the board will not proceed. Check the furnace fuse in the 12V panel. Then check the thermostat itself: on many systems, jumping or bypassing the thermostat call wire proves whether the thermostat is the dead link. Weak batteries in a digital thermostat cause more no heat calls than most people would believe.
Symptom 2: Fan Runs, Never Tries to Light
The blower spins but you never hear the click of ignition. This is the classic sail switch failure. The sail switch is a small paddle in the blower airstream that closes only when airflow is strong enough for safe combustion. Two things stop it: weak airflow and a stuck switch. Weak airflow comes from low battery voltage (a slow blower does not push the sail closed) or blocked ducting and returns, so check voltage and every register and cold air return first. If airflow is good and the furnace still never attempts ignition, the sail switch itself is stuck or failed, which is a common and inexpensive part. Behind the sail switch in the chain sits the limit switch, a thermal safety that locks the furnace out if it has been overheating; a furnace that used to short cycle before dying entirely points there.
Symptom 3: Clicks but No Flame
You hear the spark clicking, but no whump of ignition, and the furnace locks out after two or three tries. Now you are at the gas side. Confirm propane is flowing (stove test), and if the rig has been sitting, expect air in the line; cycling the stove burners for a minute purges it, and the furnace may need two or three lockout reset cycles before gas reaches the burner. If gas supply is confirmed and it still will not light, the igniter electrode is next: the gap should be roughly 1/8 inch, and cracked ceramic on the electrode shorts the spark to ground. On Dometic and Suburban furnaces the electrode assembly usually includes the flame sensor, and replacing the assembly is a normal maintenance repair.
Symptom 4: Lights, Then Dies
The furnace ignites, runs briefly, and shuts down. If it dies within seconds, the flame sensor is not proving the flame: a carboned or misaligned sensor rod is the usual cause, and cleaning it gently with fine abrasive restores the signal. If it runs a few minutes and quits, the limit switch is tripping on genuine overheat, and the cause is nearly always restricted airflow: closed registers, crushed ducting, or a return blocked by storage. A furnace needs every duct open; closing registers to push heat around the rig starves it and cooks the heat exchanger.
What Not to Do
Do not bypass the sail switch or limit switch to force the furnace to run. Those two switches are the only things standing between a fault and a furnace running without safe airflow. Diagnose which link failed and fix the link.
Get Instant Help With Your Furnace
The ignition chain is the same everywhere, but the fault codes, part locations, and access panels are model specific. Text FixMyRV.ai with your RV year, make, model, and furnace brand, and tell us which symptom above matches yours. Our diagnostic service, backed by over 50,000 pages of professional repair manuals, will take you to the exact component in your unit.