RV Water Heater Won't Light on Gas

You flip the water heater switch on the inside panel, hear nothing from outside, and a minute or two later the red light comes on. Or it clicks, maybe you hear a whoosh, and then it goes quiet and the light comes on anyway. Either way there’s no hot water on gas.

In order of likelihood on a Suburban SW or Atwood GC series heater: the tank is empty or in bypass and the ECO tripped, there’s air in the line after a tank swap, the electrode is dirty or the gap has drifted, the burner tube has a mud dauber nest in it, 12 V isn’t reaching the board, and only after all of that the board or gas valve. Most people find it within the first three steps, and the first one costs nothing.

You need a multimeter, a screwdriver, a flashlight, and something to measure a 1/8 in gap. Everything is behind the exterior water heater door except the switch and the bypass valves.

Also check: was it winterized?

The most common spring call. When you winterize, the water heater bypass valves (one, two, or three valves behind the tank, inside the rig) are turned so antifreeze skips the tank, and the tank is drained. If those valves are still in bypass in April, the tank is empty. Switch the heater on and the burner heats a dry tank, the ECO trips within minutes, and the red light comes on.

Put the valves back to normal, open a hot tap, and let it run until the flow is steady with no air. On a 6 gal tank that’s a couple of minutes on city water, longer on the pump. Then switch the heater on. The winterizing guide walks the valve positions by brand.

How a DSI water heater lights

Suburban SW6DE/SW10DE and Atwood GC6AA/GC10A series heaters (Dometic-branded after 2014) follow one chain.

1. The inside switch sends 12 V to the circuit board through the thermostat and the ECO, wired in series on the tank face.
2. The board sparks the electrode and opens the gas valve for a trial of about 7 to 10 seconds.
3. The electrode senses flame by rectification. If it sees flame, the burner runs until the thermostat opens at roughly 130°F to 140°F.
4. No flame sensed: valve closes, board waits, tries again. Three tries, then lockout, and the red light on the inside switch comes on. Switch off for 10 seconds and on again to reset.

So: red light with no click or spark means the board never got power or never got a closed ECO. Red light after clicking and sparking means no gas, no ignition, or no flame sensing.

Step 1Confirm 12 V reaches the board

Open the exterior door. The board is in a small enclosure near the gas valve. With the inside switch on, read DC volts across the board’s 12 V input terminals (usually marked + and − or 12 V and GND).

• 12 V or better: power is fine, go to step 2.
• Nothing, and no light at all on the inside switch: check the fuse at the 12 V panel (often 10 or 15 A, labeled WH or WATER HEATER) and the switch itself.
• 12 V at the switch but not at the board: a break in the wire run, or the thermostat/ECO circuit is open. That’s step 2.

The water heater board draws little current, so a weak battery that won’t run the furnace will still light the water heater. Low voltage is rarely the cause here.

Step 2The thermostat and ECO pair

Behind a small cover on the tank face (Suburban) or in a plastic housing (Atwood) are two thermal switches stacked or side by side: the thermostat, which opens at the set temperature, and the ECO (energy cut-off, also called the hi-limit), which opens at around 180°F if the thermostat fails or the tank runs dry. Both are normally closed. Both must be closed for 12 V to reach the board.

Disconnect the wires and read continuity across each at a cool tank.

• Both closed: fine, go to step 3.
• Thermostat open at a cool tank: failed; replace.
• ECO open: it tripped. On a Suburban the ECO has a small reset button in the center; press it and recheck continuity. On Atwood heaters the ECO on most models is a non-resettable thermal cutoff; once it opens, the thermostat/ECO assembly is replaced as a unit (Atwood 91447 covers many 12 V/120 V models; verify yours).

Before you reset or replace, find out why it tripped. A dry tank is the usual reason. A thermostat that’s stuck closed is the other, and it will trip the new ECO too. A heater with the tank full that trips the ECO repeatedly is a stop-box condition.

Atwood thermal cutoff. Separate from the ECO, Atwood heaters have a small inline thermal fuse on the 12 V wire to the gas valve, near the burner. It opens if the burner compartment overheats, which happens when the flue is blocked, a spider web disturbs the flame, or the flame rolls out of the burner tube. If it reads open, clear the burner and flue (step 5) and replace the fuse with the kit (Atwood 93866, two per pack). Do not jumper it or replace it with wire.

Step 3Gas supply and air in the line

If the board sparks and the valve clicks but there’s no flame, confirm gas.

• Tank open and above a quarter full. Light a stove burner and look for a strong blue flame. Weak or yellow means a supply problem, not a water heater problem.
• Air in the line. After a tank swap or any work on the system, the three tries are used up before the air reaches the burner. Run a stove burner for 30 seconds, switch the heater off for 10 seconds, and back on. Sometimes it takes two or three cycles. This is the second most common reason a heater “won’t light.”
• Excess flow valve. A tank valve opened quickly after a swap can trip the excess flow device in the pigtail; the stove will light on the trickle, the water heater’s 8,800 to 12,000 BTU/h burner won’t. Close the tank valve, wait a minute, open it slowly.
• Regulator. Two-stage RV regulators deliver 11 in water column at the appliance. If the stove weakens when the water heater or furnace calls, the regulator is suspect.

Step 4Electrode gap and condition

The electrode is a ceramic-insulated rod with its tip at the mouth of the burner tube, with a second rod or the burner itself as ground. It sparks the gas and then senses the flame. Shut the gas off and pull the electrode assembly (one or two screws).

• Gap: 1/8 in between the electrode tip and ground on both Suburban and Atwood units. Use a feeler gauge or the shank of a 1/8 in drill bit. The gap opens up over time and the spark weakens or jumps to the wrong place.
• Tip: scale, rust, or soot on it. Clean with fine abrasive. Don’t file it down; just bright.
• Ceramic: any crack means the spark is leaking to ground before it reaches the tip. Replace the electrode.
• Wire: the high-voltage lead from the board must be seated at both ends with no chafing against metal. Watch the spark with the gas off: it should jump the gap in a steady blue-white snap, not wander along the ceramic.

A heater that lights and then drops out within a few seconds, every time, is almost always a dirty electrode or a poor ground between the burner and the tank frame. Clean the mounting surfaces to bare metal.

Step 5Burner tube and flue

Look down the burner tube with a flashlight and check the flue (the U-tube on Atwood, the straight tube on Suburban). Mud daubers build in the burner tube every season, and a nest the size of a thumb is enough to starve the burner or push the flame out of the tube toward the electrode and the thermal cutoff. Spider webs across the air shutter do the same.

Pull the burner, brush it, blow it out, and clear the flue. On Atwood units, the air shutter position is set at the factory; if it’s been moved, the manual gives the opening for your model. Reinstall so the burner tube is centered in the flue opening. A correct flame is blue with a steady roar and no yellow tips lapping out of the tube.

Step 6Flame sensing

If the burner lights, runs 2 to 10 seconds, and drops out, repeating until lockout, the board isn’t seeing the flame. That’s the electrode tip, the ground path, or a flame that’s too weak to rectify (low gas pressure, dirty burner). You’ve already cleaned the electrode in step 4; now check that the board’s ground wire is tight and clean at the frame, and that the burner assembly is bolted to bare metal. Then rerun the stove test for pressure.

Step 7Pilot-light models

Atwood G6A-7 and G6A-8 and older Suburban SW6P heaters have a standing pilot with a thermocouple instead of a DSI board. Hold the gas valve knob in on PILOT, light the pilot with a long lighter through the access hole, and hold for 30 to 60 seconds before releasing. If it won’t stay lit:

• Pilot flame small or yellow: the pilot orifice is partly blocked. Clean with alcohol and compressed air, never a wire.
• Pilot lights, dies when you release the knob: the thermocouple isn’t making enough voltage to hold the valve. A good thermocouple in the pilot flame generates around 25 to 30 mV; below 10 to 15 mV the valve won’t hold. Make sure the pilot flame fully covers the thermocouple tip, then replace the thermocouple if it still drops. They’re a standard part and screw into the valve.
• Pilot stays lit but the main burner won’t come on: the thermostat is open or the valve’s main coil has failed.

Step 8The board and gas valve, last

If 12 V is at the board with the ECO and thermostat closed, the electrode is clean and gapped at 1/8 in, the burner and flue are clear, the stove burns strong, and the board still never sparks or never opens the valve, the board is suspect. Replacement is OEM or a Dinosaur Electronics UIB S, which fits most Suburban and Atwood DSI heaters; match it from their cross-reference.

To separate board from valve: during the trial for ignition, read DC volts across the gas valve terminals. 12 V present and no click or gas: the valve coil has failed. No 12 V while the electrode sparks: the board isn’t commanding the valve. Replacing the valve means opening a propane connection and leak-testing with soap solution or a leak detector afterward.

Symptom → likely cause

Parts and tools mentioned

• Digital multimeter with DC volts, millivolts, and continuity
• Suburban 12 V thermostat/ECO kit (232306 on many SW models; verify)
• Atwood thermostat/ECO assembly (91447 on many models; verify)
• Atwood thermal cutoff kit (93866)
• Replacement electrode for your model
• Thermocouple for pilot models
• Dinosaur Electronics UIB S or OEM board
• Feeler gauge or 1/8 in drill bit, soft brush, compressed air, isopropyl alcohol
• Soap solution or propane leak detector

Questions people ask next

What does the red light on the water heater switch mean? Lockout. The board tried three times to light and gave up, or the ECO/thermostat circuit is open so it never got power. Switch off, wait 10 seconds, switch on. If it comes back on within two minutes, start at step 1.

Can I reset the ECO on an Atwood? On most Atwood models, no. The ECO is a one-time thermal cutoff and the thermostat/ECO assembly is replaced. Find out why it tripped first; a dry tank is the usual answer. Suburban ECOs have a reset button.

The heater lights on electric but not gas. Is it the board? Probably not. Electric and gas share the tank but nothing else; the 120 V element has its own thermostat and ECO. A heater that works on electric and won’t light on gas points at the gas side: supply, electrode, burner, then board.

Why does it only fail after I swap tanks? Air in the line. The water heater sits at the end of a long run and the three tries are used up before gas arrives. Purge at the stove and recycle the switch. If it happens even without a swap, check the excess flow valve and the regulator.

It lit fine all summer and quit in the first cold snap. Check the tank level and the regulator. A nearly empty cylinder in cold weather can’t vaporize fast enough to hold pressure once the furnace and water heater both call. The propane usage guide explains why.