Tankless problems, sorted by symptom

Tankless error codes look precise and travel badly. A two-digit code on one brand is an ignition fault. The same number on another brand is a fan fault. The same brand reuses numbers when it changes the control board.

Copying a chart from a different generation sends a technician to the wrong part and, worse, sends a homeowner into a gas valve. This page sorts by what the water and the fan do. You still write the code down.

You match it to the manual for the serial number on the side of the cabinet, from the manufacturer’s support site, not from a forum signature. How the machine is supposed to behave is in how a tankless heater works.

Stop. Gas odor, a burner that will not stop, or a cabinet that is hot and smelling of scorched wire means shut the unit off at its switch, shut the gas if the valve is reachable without forcing anything, and call. Do not keep tapping reset. Several ignition retries can fill a combustion chamber with unburned gas.

Nothing happens when you open a tap

Confirm the complaint is the heater. Other fixtures in the house should be checked. A single dead faucet is a faucet.

Then check power. Tankless gas heaters need electricity for the fan and the control.

A tripped breaker or a GFCI that tripped after a storm produces a dark display. Reset once. If it trips again, leave it off and read electrical safety.

Confirm the controller is not set to off or to a vacation mode, and that the set point is not below the incoming water temperature, which on some units means “do not fire.”

The next threshold is minimum flow. Many models will not start below roughly 0.4 to 0.6 gallons per minute. A low-flow lavatory faucet, especially with a clogged aerator, can sit under that line.

Open a tub filler, which is a higher flow, and see whether the heater wakes. If the tub filler wakes it and the bathroom sink does not, clean the aerator or accept that the sink is below the activation flow.

The manual states the minimum for that model. Do not defeat the flow sensor. It is what keeps the heater from firing with no water to carry the heat.

If the display is alive, the flow is generous, and the fan never starts, the unit is not seeing a call for heat or the fan circuit is dead. That is a technician’s diagnosis.

Inlet screens clog with debris after plumbing work and can drop the flow the sensor sees. Cleaning a screen the manual shows, with the water and power off, is a fair homeowner step. Opening the gas valve body is not.

The fan runs and the water stays cold

The heater is trying. Ignition, gas supply, or an exhaust switch is stopping it. Confirm the gas cock serving the unit is open and that another gas appliance in the house lights.

If nothing gas works, call the utility. If the range lights and the heater clicks and locks out, write down the code and stop retrying.

Outdoor terminations blocked by snow, leaves, or a stored trash bin produce exactly this symptom on sealed-combustion units. If you can see the termination from the ground, clear it.

Do not climb onto a roof, and do not lean over a balcony to reach a vent. A bird screen packed with lint is a service item if it requires removing the vent.

Condensate drains that are frozen or clogged also prevent ignition on condensing models, because the pressure switch will not prove.

A hose you can see, popped off a barb, can be put back. The chemistry and the trap are in condensing gas heaters.

Inadequate gas pressure under full fire mimics an ignition or a performance fault. The line may be fine for a cooktop and too small when the water heater asks for 199,000 Btu/h. A manometer on the inlet, while the unit fires, is the test.

The data plate states the pressure. Adjusting the gas valve to chase a sagging supply overfires the heater or starves it and produces carbon monoxide. Pipe size and regulator capacity are a fitter’s job.

Lukewarm water, especially in winter

If the unit used to keep up in September and fades when two showers run in January, do the rise arithmetic before you replace the heat exchanger. Incoming water got colder, so the same burner yields fewer gallons per minute at the same set point. Sizing a tankless heater works the Department of Energy example and the winter adjustment. A unit that is truly undersized will be lukewarm at high flow and fine at a trickle, which is the opposite of a minimum-flow problem.

Scale produces the same lukewarm result all year and adds noise. Descaling through the service valves is the maintenance procedure. If there are no service valves, the first repair is to add them, then descale.

A heater that has never been descaled on a hard well can be scaled past the point where vinegar circulation restores full flow.

The technician’s next test is whether the heat exchanger still holds pressure. A failed exchanger is a replacement or a major part, and repair or replace applies. Hardness is in hard water and scale.

Hot, then cold, then hot

That is the cold-water sandwich: water that cooled in the exchanger and the pipe between draws, followed by heated water once the burner catches. It is worse at fixtures far from the heater. It is not, alone, a failed part.

Recirculation, with its energy cost, is the comfort fix and is described on the how-it-works page. A sandwich that arrives as a long lukewarm stretch, not a brief slug, is more likely scale, a crossover, or an undersized unit.

Shut the heater’s outlet valve. If a hot faucet still runs, cold water is crossing into the hot line through a cartridge or a stuck recirculation valve. Fix the cross. Do not buy a larger heater to feed it.

Codes that return immediately

A code that clears and returns on the next draw is a real fault. Resetting it through an evening is how igniters, gas valves, and people get damaged.

Record the code, the model, the serial number, what the fan did, and whether any faucet was unusual. Then call.

Give the technician the maintenance history. “Never descaled, fifteen-grain well, code appears at high flow” is a useful sentence. “It beeps” is not.

Symptom to system, not to a part number

What you notice	Often involved	Homeowner limit
Dark display	Power, GFCI, internal fuse	Reset the breaker once. Stop if it trips.
Display on, no fan, no heat	Set point, flow sensor, minimum flow, controller	Try a high-flow tap. Clean a listed inlet screen.
Fan runs, no flame, code	Gas supply, igniter, exhaust or intake, condensate	Clear a termination you can reach safely. Then stop.
Lukewarm only at high flow or only in winter	Sizing and groundwater, or scale	Do the rise math. Descale if maintenance is overdue.
Brief hot-cold-hot slug	Normal sandwich, or a crossover if it is long	Consider recirculation. Test for a cross before replacing the unit.
Whistle or kettling	Scale	Descale. Shut down if the noise is a bang.
Code returns every draw	Whatever the manual says for that code	One reset to confirm. Then a technician.
One faucet only	Cartridge, aerator, or that branch	Do not replace the heater.

What the manual is for

Rinnai, Navien, Rheem, Noritz, Bosch, and the other makers each publish the code list for a given model. Download the PDF that matches the model and, when the site asks, the series.

Read the ignition and venting sections before you assume a part. Some codes require a licensed tech by the manufacturer’s own warranty terms, and a homeowner reset can void the labor coverage.

The warranty questions to ask are listed in how long heaters last. Keep the manual where the next person can find it. A photo of the data plate in your files is enough.

Sources

U.S. Department of Energy, Energy Saver, “Sizing a New Water Heater,” for flow and temperature-rise sizing that explains winter lukewarm water. U.S. Department of Energy, Energy Saver, “Which Water Heater Is Right for You?,” for the output limits of demand heaters.

Manufacturer support libraries for model-specific codes: Rinnai (rinnai.us), Navien (navieninc.com), Rheem (rheem.com). Installation manuals for minimum activation flow, service valves, and vent clearances. Do not use a code chart from a different model year.