The fault is real. The failure it names usually is not
An on demand LP water heater does not have a temperature dial and a thermostat that either closes or does not. It has a control board reading a flow sensor, an inlet temperature sensor, an outlet temperature sensor, a flame sensing electrode and battery voltage, all at once, and it modulates the burner against those inputs many times a minute. When any one of those readings falls outside a window, the board shuts the burner down and lights the indicator.
Which means the fault is a report about a sensor reading, not a diagnosis of a part. Nine times out of ten the sensor is telling the truth and the thing it is measuring has been changed by something upstream. Owners who read the indicator as an instruction to replace a component buy a board or a sensor, install it, and get the identical behavior within a week.
The practical version: on a unit that has been in service in this area for three or more seasons, treat every fault as a water quality question first and a hardware question second. That single reordering saves most of the money spent on these units.
What the unit does when scale bridges a temperature sensor
The specific behavior owners describe is a burst of hot water, then lukewarm, then the indicator, and then normal operation again twenty minutes later. That is a thermal runaway trip, and it happens when scale has insulated the outlet sensor or narrowed the passage in front of it. The board sees a temperature spike that the water never actually reached and protects itself.
The second pattern is a unit that will not fire at low flow. On demand heaters need a minimum flow rate before the board authorizes ignition. Scale in the flow path or a partially blocked inlet screen drops actual flow below that threshold, so a low pressure faucet gets cold water and a wide open shower works fine. Owners read that as a sensor failure. It is a restriction.
The third is intermittent lockout that tracks battery state. Below roughly 11 volts the board can reset mid cycle and log an ignition fault that has nothing to do with gas or ignition. If your unit misbehaves only on the second or third day off shore power, chase the house bank before you touch the water heater.
San Gabriel Valley water is the actual defendant here
Groundwater across the San Gabriel Valley is genuinely hard, routinely far above the 120 milligram per litre threshold at which water gets classified that way, and it is the water going into most coaches filled in and around Hacienda Heights. Hardness that a house never notices is a different matter inside a heat exchanger that raises water temperature across a few inches of passage.
An instantaneous heater is the worst case for scale by design. There is no tank where minerals settle to a bottom you can flush. Water passes through a narrow finned exchanger directly over a burner, so the film on the hot side of the passage is where calcium drops out, precisely on the surfaces that need to transfer heat and precisely where the sensors read. Deposits that would take a residential tank a decade to build accumulate here in two or three seasons.
Then add the storage pattern. A coach that sits in an uncovered lot between trips holds still, hot, mineral rich water in the exchanger through a Southern California summer, and still water at temperature deposits faster than moving water. The units we see in the worst condition are not the ones that travel constantly. They are the ones that get used one weekend a month and parked.
A descale at $350 against $4,500 of exchanger work
Caught at the first fault. A descale cycle, an inlet screen cleaning and a sensor verification sit at the bottom of the water heater repair and replacement band, $350 to $4,500 across 1 to 18 hours. That is the entire fix on most units that have never been serviced, and it usually returns full output the same day.
Caught after two seasons of faults. Sensors that have been scaled and thermally cycled get replaced along with the descale, and the LP side gets verified because a unit that has been short cycling has been running its burner outside its intended pattern. LP gas system service runs $285 to $2,500 across 1 to 10 hours, and there is a $45 flat hazmat and disposal charge on any job where LP is involved.
Caught after the exchanger is packed. Once scale has bridged the passage rather than coated it, chemical descaling will not clear it and the exchanger or the whole unit gets replaced. That is the top of the same $4,500 band, plus fresh water system repair at $285 to $4,500 if the supply plumbing has to be reworked to fit the replacement. Systems and electrical jobs run 3 business days to 2 weeks.
The Truma channel, and why the exact model variant matters
Truma parts come through a controlled distribution channel rather than off a general RV shelf, and the units are not interchangeable across variants. There are meaningful differences between the AquaGo product levels in how descaling is handled, in the control board, and in the plumbing interface, and a part that physically bolts up can still be electrically wrong. The model and serial data plate is the order document, not the coach brochure.
Consumables and service items, the descaling solution, inlet screens, gaskets and electrodes, generally land inside a week. Control boards and exchanger assemblies are where the calendar moves, typically 2 to 4 weeks and longer when a variant has been superseded. We check availability against your data plate during evaluation so the calendar we quote is the one you get.
The cost note that surprises people: special order parts carry a 100 percent deposit at order and that deposit is non refundable. On a control board that is real money, which is another argument for confirming the diagnosis before ordering anything.
Why this almost never becomes an insurance claim
Scale is wear. It accumulates gradually from ordinary use of ordinary water, and every policy excludes it in language that has been tested for decades. Filing on a scaled exchanger produces a denial and a paper trail you did not need.
What is covered is a sudden event, and there are two worth knowing. A freeze split, if the unit was not drained before a cold snap, is generally covered as sudden physical damage, though some policies treat failure to winterize as neglect. And water damage downstream of a burst fitting is usually covered even when the fitting itself is not, which is the same logic that applies to a broken pipe in a house.
The one that pays and gets missed is collision. If the coach was struck on the side where the heater lives, the unit, its plumbing and its LP run belong in the collision scope. That gets written at teardown, priced at 1 hour at $210 and credited to an authorized repair.
This is certification gated work, and the DIY line is short
Anything that opens the gas side of this unit is certification gated and is performed by the certified technician in the shop. That includes the gas valve, the manifold, the orifice, the regulator side of the connection and any leak down verification. The reason is not paperwork. An LP joint that is reassembled by feel and never manometer tested is a sleeping problem inside an occupied vehicle.
The safe owner side is genuinely useful. You can run the manufacturer's descale routine on a unit that supports it, clean the inlet screen, drain the unit before storage, check house battery voltage, and record what the indicator does and when. All of that is water side and electrical monitoring, and doing it on a schedule is what keeps you out of the $4,500 band.
What you should not do is pull sensors, jumper the flow switch, or open the burner enclosure to look at the exchanger. On the units we get in worst shape, a previous attempt at one of those three things is already visible.
Log the fault, run the water, then call the shop
Do three things this week. Record the exact indicator behavior and what the water was doing when it appeared, including which fixture was open and how far. Time how long a wide open shower takes to go hot, then repeat at a half open bathroom faucet, because the difference between those two is the flow threshold telling you about restriction. Check resting house battery voltage after a day off shore power.
Then bring it in. We are 18.4 miles from Hacienda Heights, 24 to 38 minutes via SR-60 west to Fullerton Road, or SR-60 to SR-57 south and SR-91 east if the corridor is stacked. All LP and water heater work happens in the bay at the Yorba Linda facility. Call (949) 799-3387 with the model and serial off the data plate. The RV systems estimate is $150 and it is credited to an authorized repair, and an in depth diagnostic if the fault turns out to be electrical is 1 hour at $285.