THE SHORT VERSION
- Write the code down before you do anything. It names the fault, and it’s the first thing a technician will ask for. The same fault has a different code on each brand: failed ignition is 11 on Rinnai, Rheem and Noritz, E003 on Navien and 111 on Takagi.
- Reset once. If the same code comes back, stop. The usual causes, such as gas pressure, venting, the igniter or the flame sensor, are technician work, and the manufacturers’ own code tables say so.
- What you can safely check: the gas valve is open, the unit has power, the tap is open far enough (most units need about 0.4–0.5 gallons per minute to fire), the cold inlet filter is clean and the outside vent isn’t buried in snow or leaves.
- Scale codes (Rinnai LC or SS, Rheem 1L, Navien E760) mean the heat exchanger needs flushing. Damage from scale, poor water quality and freezing is excluded from the warranties we read.
- Freeze protection needs electricity. In a power outage in freezing weather, Rinnai, Navien, Noritz and Takagi all say to drain the unit.
Start with the code: write it down, reset once
A gas tankless water heater monitors water flow, flame, temperatures and exhaust. When something is out of range it shuts the burner off and shows a code. Photograph the code before you press anything.
Then try one reset. How you reset depends on the brand and on how serious the fault is:
- Navien NPE-2: press the Back button on the front panel, its user’s manual says, for minor problems.
- Noritz: “soft” lockouts clear on their own once the condition passes. “Hard” lockouts, which on its EcoTough models include ignition failure (11), the condensate line (29), gas leakage (40) and flame (72), are cleared by turning the Power button off and on.
- Takagi: most codes in its 240HX3–540HX3 manual clear when water flow stops.
- Rheem: its service manual reserves a separate reset procedure for hard lockouts (codes 10, 13 and 99) that starts with removing the front cover. That one is for a technician.
- Rinnai: its Sensei manual stores the last nine codes in a diagnostic history, so a technician can see what happened even after a reset.
If the same code returns, leave it. Rinnai’s manual puts a warning over its code table that some checks “should be performed by a trained and qualified professional,” and most rows in Navien’s table simply say to call technical support. Resetting a safety lockout over and over doesn’t fix the cause and can hide a dangerous one.
What you can check yourself, and what you can’t
The checks the manufacturers give owners are all outside the case: gas valve, power, water flow, filters, the vent ends and the condensate drain. Everything behind the front cover runs on gas and household voltage and belongs to a technician.
The same fault has a different code on each brand
| Symptom | Rinnai | Navien NPE-2 | Noritz | Rheem | Takagi HX3 |
|---|---|---|---|---|---|
| No ignition | 11 | E003 | 11 | 11 | 111 |
| Flame lost while running | 12 | E012 | 12 | 12 | 121 |
| Overheat shutdown | 14, 16 | E016; E030 (exhaust over 149°F) | Not in the guide we read | 14, 16 | 941 (exhaust temperature) |
| Air or vent problem | 10 | E110 | 29 (EcoTough), 90 | 10, 90, 99 | 991, 101 |
| Condensate not draining | 10 (Sensei), 25 (pump) | Not listed | 29 (EcoTough) | 29 | 291 |
| Scale or service due | SS (Sensei); LC (older condensing) | E760 | Not listed | 1L | 661 (bypass valve stuck with scale) |
| No heat, no code | Flow under 0.4 gpm | Flow under 0.5 gpm | Flow under 0.5 gpm | P1, low flow | Flow under 0.5 gpm |
Sources: Rinnai RSC (Sensei) manual and Technical Bulletin 75 (2010); Navien NPE-2 user’s information manual; Noritz EcoTough owner’s guide and Noritz’s 2022 error-code article; Rheem SVC820P tankless troubleshooting pocket manual; Takagi 240HX3/340HX3/540HX3 manual. Codes differ between product lines of the same brand. Use the manual for your model number.
No ignition: codes 11, E003 and 111
The unit sensed water flow, tried to light and never saw a flame. Noritz explains that ignition needs three things, air, spark and gas, and that this code means one of them is missing. The common culprits it lists are an empty propane tank, a disrupted gas supply, an airflow problem or debris on the igniter.
Safe checks
- Is the gas valve on the line to the heater open? Navien’s only owner action for E003 is to make sure the main gas supply valve is open.
- Do other gas appliances work? If the stove won’t light either, the problem is the gas supply. Call the utility or your propane supplier.
- Is there propane in the tank? Rinnai’s first checks for codes 11 and 12 include gas at the meter or cylinder.
- Is the display on? No power means no fan and no spark.
Past that, it is a technician’s job. Rinnai’s list for code 11 includes gas type and inlet pressure, air in the gas line, the igniter, the flame rod wire and the gas valves; Noritz says to measure gas pressure with a digital manometer. On a new install, ask whether the gas line and meter were sized for the heater, a check Rinnai’s bulletin lists for both codes 11 and 12.
Flame goes out: codes 12, E012 and 121
The burner lit, then lost its flame. Rheem’s service manual says the cause is “commonly GAS SUPPLY and/or VENTING.” Noritz adds a pattern worth noticing: on propane, a low tank can cause flame loss, and an undersized gas supply shows up when several gas appliances run at once, as in winter when the furnace is on.
- Note when it happens. Only with the furnace or dryer running? Only at high flow, like two showers? Only on cold mornings? That pattern points a technician to gas supply or venting.
- Check the propane level if you have a tank.
- Call if it recurs. The fixes include gas pressure, line sizing, the flame rod and venting, all pro work.
Overheat shutdowns: scale, flow or combustion
Overheat codes come from safety switches and sensors that cut the burner when the heat exchanger, outgoing water or exhaust gets too hot. Navien’s E030 trips when the flue temperature exceeds 149°F. Rinnai’s code 14 (heat exchanger overheat) tells technicians to look for hot spots that indicate blockage from scale, and Rheem’s code 14 is “normally caused by inadequate/wrong GAS SUPPLY and/or VENTING.”
For an owner, the one practical step is the one Navien gives for E030: clean the cold-water inlet filter, which its manual walks you through after shutting off and draining the heater. Its other remedy is flushing the heat exchanger. Then reset once. Any overheat code that returns needs a technician, because these devices are the last line of protection against a damaged exchanger.
Cold water and no code: usually too little flow
A tankless heater only fires when it senses enough water moving through it. Below that, it doesn’t start, and no code appears. Noritz points out that without flow there is nothing to trigger an error. This is the classic “cold at the bathroom sink, fine at the shower” complaint.
| Brand and model line | Flow needed to start | Source |
|---|---|---|
| Rinnai gas units | 0.4–0.6 gpm to ignite; 0.26 gpm to keep running | Rinnai residential FAQ |
| Navien NPE-2 | 0.5 gpm | NPE-2 user’s information manual |
| Noritz EcoTough | Over 0.5 gpm | EcoTough owner’s guide |
| Takagi HX3 | Over 0.5 gpm | 240HX3–540HX3 manual |
gpm: gallons per minute. A WaterSense bathroom faucet is rated at 1.5 gpm or less at full flow, so a faucet opened a little can fall below these minimums.
- Open the hot tap further, or use a fixture with more flow, and see whether the unit fires.
- Clean the aerator on the faucet and the showerhead screen. Rinnai and Rheem both list this.
- Clean the cold-water inlet filter on the heater, following your manual. Rinnai, Navien, Noritz and Takagi all name it.
- Lower the temperature setting a few degrees. Rinnai explains that a lower setting makes you open the hot side further, so more water flows through the heater.
- On a brand-new install, ask the installer to check that the hot and cold connections aren’t reversed, which Rinnai, Noritz and Takagi each list.
If water turns lukewarm when you open a cold tap elsewhere, or a single-handle shower runs only cold, ask about a plumbing crossover, where hot and cold mix inside a faulty fixture. Rheem’s service manual includes a test for it. A short burst of cold between two hot draws is a different, normal effect; see the cold water sandwich.
Scale warnings: flush the heat exchanger
Hard water leaves mineral scale on the hottest surfaces, which in a tankless unit is a narrow heat exchanger. Some units warn you before it causes damage:
- Rheem 1L: the control has detected lime build-up and the unit “must be drained and flushed,” sometimes more than once.
- Rinnai LC (older condensing models): scale build-up in the heat exchanger. Rinnai’s bulletin says it is the only code that lets the unit keep running, and it resets if power is cycled. That makes it easy to clear without fixing anything.
- Rinnai SS (Sensei): a time-based “Service Soon” reminder the installer can set.
- Navien E760: flushing or service alarm; Navien says to contact the installer for routine service.
Don’t just clear the warning. Rinnai’s manual says “damage and repair due to scale in the heat exchanger is not covered by warranty,” and Noritz excludes damage from poor water quality, asking owners to treat water harder than 200 mg/L (12 grains per gallon). Flushing means circulating a descaling solution through service valves with a small pump; Navien calls it “a somewhat complicated procedure” and warns that improper maintenance can void the warranty. See descaling a tankless water heater and hard water and your water heater.
Vent, air and condensate codes
Tankless heaters use a fan to push exhaust out and pull combustion air in. When a pressure switch or sensor sees the vent restricted, or the condensate drain backed up, the unit shuts down. Rheem’s 90 and 99 codes, for instance, mean the unit detected a blocked vent before or after a burn. On condensing models, Rinnai’s code 10 covers “Air Supply or Exhaust Blockage/Condensate Trap is Full.”
Safe checks for vent and condensate codes
- Look at the vent ends outside. Snow drifts, leaves, nests and wasps block them; clear them by hand from outside if you can reach safely.
- Clean the air intake filter if your model has one. Navien (E110) and Noritz (29) both list it as an owner task.
- Follow the condensate hose from the unit to its drain. Kinks, sags and freezing stop it; Takagi’s manual says horizontal runs should slope downward.
- Then reset once.
A white plume from the vent in cold weather is normal; Rinnai says it is water vapor from the exhaust condensing. Soot, a burning smell, water running back out of the vent or a carbon monoxide alarm are not. Vent repair is pro-only work, because a leaking vent can put carbon monoxide in the house. See water heater venting.
Freezing: protection works only with power
Tankless units hold only a little water, but it sits in thin tubes that split when they freeze. All four manuals we read have freeze protection, and every one excludes freeze damage from the warranty.
| Brand | What protects the unit | What it needs |
|---|---|---|
| Rinnai (Sensei) | Protects down to -22°F indoor models or -4°F outdoor models, out of direct wind | Gas and 120-volt power |
| Navien NPE-2 | Freeze protection function; Navien says freeze damage most often comes from back-drafting caused by negative pressure in the building | Electricity; gas left on (A2 models) |
| Noritz EcoTough | Pump circulates water below about 46°F; burner fires below 41°F | Power, gas, water and the main switch on |
| Takagi HX3 | Heating blocks switch on below 36.5°F | Electrical power |
Sources: each manufacturer’s manual listed below. None of these protects the water pipes outside the unit; Rinnai and Takagi say to insulate them.
If you suspect the unit froze, Navien’s first steps are to turn it off and close the gas valve. Don’t run it until a technician has checked it for leaks.
When to call, and what to tell the technician
Call a licensed plumber or the manufacturer’s service line when a code returns after one reset, when any code involves gas, flame, overheating or venting and the simple checks didn’t fix it, or when you see water inside the case. Rinnai’s manual also says not to use the heater if any part has been under water.
Have this ready
- Brand, model and serial number from the rating label, and the install date
- The code, exactly as shown, and how many times it appeared
- When it happens: first use of the day, high flow, cold weather, other gas appliances running
- Gas type (natural gas or propane) and, for propane, the tank level
- What you already tried: reset, filter cleaning, vent check
- Any recent work: new gas appliance, remodel, water softener, power outage
Check the warranty before you pay for parts. Navien’s NPE-2 warranty, for example, runs 15 years on the heat exchanger, 5 years on parts and 1 year on labor for standard residential use, but excludes damage from hard water deposits and gas flow problems. See water heater warranties. If the heat exchanger itself has failed outside warranty, weigh the repair against a new unit with repair or replace.
Common questions
What does error code 11 mean on a tankless water heater?
On Rinnai, Rheem and Noritz units, 11 means the burner failed to ignite. Check that the gas valve is open, other gas appliances work and a propane tank isn’t empty, then reset once. If it returns, a technician needs to check gas pressure, the igniter and the flame sensor.
Can I just reset my tankless water heater?
Once. A reset clears the code but not the cause. If the same code comes back, call a technician; some lockouts, such as Rheem’s hard lockouts, can only be reset with the front cover off.
Why does my tankless water heater not turn on at low flow?
It needs a minimum flow to fire: 0.4–0.6 gallons per minute for Rinnai and about 0.5 for Navien, Noritz and Takagi. Open the tap further, clean aerators and the inlet filter, or lower the temperature setting so you open the hot side more.
How do I clear an LC code on a Rinnai?
Cycling the power clears it, but LC means scale has built up in the heat exchanger, and Rinnai says to flush it. Clearing it without flushing leaves the scale in place, and Rinnai’s warranty doesn’t cover scale damage.
Is white smoke from my tankless vent a problem?
Usually not. Rinnai says it is water vapor from exhaust condensing in cold air. Soot, a smell, water dripping back out of the vent or a carbon monoxide alarm are different: get out if the alarm sounds, and call a pro.
Will my tankless water heater freeze in a power outage?
It can. Freeze protection needs electricity, and Rinnai’s also needs gas. Rinnai, Navien, Noritz and Takagi all say to drain the unit if it will be without power in freezing weather, and none of them covers freeze damage under warranty.
Does a tankless water heater error code mean it needs replacing?
Rarely on its own. Most codes point to gas supply, venting, flow, scale or a replaceable part such as a sensor or igniter. A leaking or cracked heat exchanger is the expensive failure; check its warranty before deciding.
Sources
Sources checked October 5, 2026. Prices, codes and rebates change. Check current details with your installer and local permit office.
- Tankless water heater installation and operation manual 100000773 (RSC/Sensei) ↗ · Rinnai
- Technical Bulletin 75: condensing water heater error codes (August 2010) ↗ · Rinnai
- Residential FAQ (minimum flow, error codes, freeze protection, white exhaust plume) ↗ · Rinnai
- NPE-2 condensing water heaters user’s information manual ↗ · Navien
- EcoTough NRCP1112-DV / NRCP982-DV owner’s guide ↗ · Noritz, hosted by TanklessParts
- Common Noritz tankless error codes: updated (June 2022) ↗ · Noritz
- Tankless gas troubleshooting pocket manual SVC820P ↗ · Rheem, hosted by ABR Wholesalers
- On-demand water heater installation manual and owner’s guide 2000599585, 240HX3/340HX3/540HX3 ↗ · Takagi
- WaterSense bathroom faucets ↗ · U.S. EPA