No hot water, won't ignite, error codes, or sulphur smell — this guide covers all four RV water heater types: tank (Atwood/Suburban), tankless (Girard), Truma Combi, and Aqua-Hot hydronic systems.
Water heater problems in RVs span a wide range — from the simple (bypass valve left open, propane tank empty) to the technical (failed ignition board, electric element burned out from running dry, Aqua-Hot glycol pump failure). The system type matters a lot because each one fails differently.
Before diving into system-specific diagnosis, there are two things every water heater complaint needs checked first: whether the bypass valve is in the correct position, and whether the unit has water in it. Both will produce a "no hot water" complaint and both are common.
Two checks before anything else.
1 — Is the water heater bypass valve in the NORMAL (not bypass) position? Winterizing leaves it in bypass, which means cold water mixes with or replaces hot. 2 — Is the tank actually full of water? Running a water heater dry blows the ECO on electric models and can damage the element.
Specific test procedures, the exact readings you should see, and what it means when something's off. The kind of detail that tells you whether this is a $12 fix or a call to your dealer.
And you won't be working through it alone. A diagnostic chatbot built on 21 years of real field experience is coming soon — so when you hit a wall, you can ask the question directly.
The ignition sequence on a tank water heater is: thermostat calls for heat → igniter sparks → gas valve opens → flame sensor confirms flame → sparking stops. A failure at any step produces the same symptom: clicking but no heat. Isolating which step failed determines whether this is a propane issue, a board issue, or a component failure.
The ECO (Energy Cut-Off) is a manual-reset safety device. It trips when the tank reaches approximately 180°F — usually from running dry, a failed thermostat, or leaving the unit on electric mode at a very hot campsite. It is located on the upper end of the water heater, behind the outer door. It has a small button in the center. Press it firmly until you feel a click — if it resets, the unit should begin heating. If it trips again quickly, there is a temperature issue to diagnose.
| Electric Mode Test | Reading | Diagnosis |
|---|---|---|
| 120V at element terminals (unit on, shore power connected) | 108–126V AC | Power is reaching element — element may be failed |
| 120V at element terminals | 0V | ECO tripped, breaker off, or wiring break — check ECO first |
| Element resistance (disconnected, meter on Ω) | 10–16Ω | Element is good |
| Element resistance | OL / open | Element burned out — replacement needed |
| Element resistance | Near 0Ω | Element shorted — replacement needed |
A rotten egg smell from hot water taps is almost always the magnesium anode rod reacting with bacteria in the water. It is not a plumbing problem and it is not a water supply problem — it is the anode rod doing its job in water chemistry that produces hydrogen sulfide gas as a byproduct.
To confirm: run cold water from the same tap. If the smell is only on hot water, it's the anode rod. The fix is to remove the anode rod (1 1/16" socket), flush the tank thoroughly, and reinstall. In areas with high sulfur content in the water supply, some owners replace the magnesium rod with an aluminum/zinc rod, which does not produce the same reaction. Others remove the anode rod entirely — not recommended long-term as the rod protects the tank from corrosion, but acceptable short-term for the trip.
The most common Girard complaint is not a failure — it is a misunderstanding of how the system activates. The unit requires a minimum flow rate (typically 0.5 GPM) before the burner fires. If water flow splits across multiple outlets, or if the hot water tap isn't fully open, the unit may not fire at all.
| Code | Meaning | First Step |
|---|---|---|
| E1 | Ignition failure — no flame detected after attempts | Check LP supply, burner blockage, electrode gap |
| E2 | Overheat — outlet water temperature exceeded limit | Check flow rate, check for scale buildup on heat exchanger |
| E3 | Cold water inlet sensor fault | Check sensor wiring at inlet; sensor replacement if wiring is clean |
| E4 | Hot water outlet sensor fault | Check sensor wiring at outlet; sensor replacement if wiring is clean |
| E5 | Fan motor fault | Check 12V supply to fan; check for debris blocking fan wheel |
| E9 | Gas valve fault — valve not responding | Check 12V supply to valve; gas valve replacement |
To clear most codes after resolving the underlying issue: turn the unit off at the switch, wait 30 seconds, turn back on. Some codes (E1, E5) require resetting the main power to fully clear.
The Truma Combi is an error-code-driven system. Unlike tank heaters where you're testing individual components, the Combi's onboard electronics identify the fault and display a code. Start with the code — don't start probing components.
Error codes display on the Truma CP Plus control panel as a flashing "E" followed by a number. The panel is inside the coach, usually in a bedroom or hallway. Press and hold the Mode button to cycle through stored codes. Note all codes before resetting — multiple codes can appear, and clearing them before noting them loses diagnostic information.
| Code | Meaning | First Step |
|---|---|---|
| E01 | Gas supply fault — no ignition | Check LP supply, regulator, gas valve position on Combi unit |
| E02 | Flame sensor fault — flame lost after ignition | Clean burner, check LP pressure, check for wind or draft at exhaust |
| E03 | Overheating — high limit tripped | Check clearances around unit, check for blocked vent, allow to cool and reset |
| E04 | Fan fault — combustion air fan not running at correct speed | Check 12V supply to fan, inspect fan for debris or blockage |
| E05 | Water temperature sensor fault | Check sensor connection at sensor body; sensor replacement if connection clean |
| E08 | Communication fault between panel and unit | Check harness connection at both ends; check for damaged wires in harness |
| E10 | Low voltage — unit shutting down to protect electronics | Check battery voltage and converter charging; unit needs 11.5V minimum |
The Combi does both jobs but prioritizes differently depending on mode. If you have heat but no hot water, or hot water but no heat, the control panel mode selection is the first place to look — not the unit itself. Verify the CP Plus is set to the correct operating mode. The unit will only run the function it's been told to run.
The Combi water heating circuit fills from a small internal tank (approximately 10 liters). If you've been running it on gas with low LP pressure, the burner may have short-cycled enough times to trigger a lockout. A full power reset — disconnect shore power and remove the Combi fuse for 60 seconds — clears most lockout states without needing a service call.
The Aqua-Hot is a diesel-fired hydronic system that heats a glycol loop. That loop circulates through zone heaters for space heating and through a water-to-water heat exchanger for domestic hot water. When you have no hot water from an Aqua-Hot, there are three places to look: the diesel burner, the glycol loop pressure, and the circulation pump.
The burner fires a sequence on startup: 1) pre-purge fan run, 2) ignition spark, 3) fuel solenoid opens, 4) flame establishes, 5) flame sensor (photocell) confirms, 6) burner runs. A failure in this sequence produces a lockout — the unit tries three times then shuts down. The amber fault light on the burner housing illuminates on lockout.
The Aqua-Hot system operates on a closed glycol loop. Correct cold pressure is 12–15 PSI. Check the pressure gauge on the expansion tank (usually mounted near the Aqua-Hot unit). Low pressure means the loop has lost fluid — there is a leak somewhere in the system. The most common locations are: hose clamp connections at the zone heater coils, the expansion tank bladder (internal failure causes pressure loss with no visible external leak), and the heat exchanger fittings.
Do not add water to the glycol loop. Only add the correct 50/50 propylene glycol/distilled water premix. Adding plain water dilutes freeze protection and promotes corrosion inside the heat exchanger.
With the Aqua-Hot running, you should be able to hear the circulation pump — a faint hum from the pump body. No hum with 12V confirmed at the pump terminals means the pump motor has failed. A hum with no flow (glycol temperature not rising evenly through the loop, some zones warm and others cold) suggests an airlock in the loop or a pump impeller worn out from running with low glycol. An air bleed at the highest point in the loop clears most airlock situations.