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Tankless Water Heater Error Code: What It Means

Last updated October 15, 2026.

Safety first: If you smell gas, leave now and call the gas utility or 911 from outside. If a CO alarm sounds, get everyone outside and call 911. If hot water or steam is spraying from the heater or its relief pipe, stay back. For a serious burn, call 911.

Write the code down exactly and look it up for your brand below. About a third of codes have a step you can do (clear a vent, open a tap wider, refill propane, one reset if your manual allows). The rest are a fault the heater caught on purpose, and the notes say why it needs a pro.

Most common codes, by what they mean

  1. Ignition / flame loss: Rinnai 11/12, Navien E003/E012, Rheem 11/12, Noritz 11, Takagi 111/121. Check gas works elsewhere and propane level; look (don’t turn) at the gas handle.
  2. Air/vent blocked: Rinnai 10, Navien E110, Rheem 90/99, Noritz 90. Clear snow or debris at the outside ends by hand. Rheem asks for 12 in. clearance at the intake (p. 75).
  3. Scale / service: Rinnai LC/00/LC0–LC9, Rheem 1L/88, Navien E760, Noritz C1#. Schedule a descale.
  4. Overheat: Rinnai 14/16, Navien E016/E030, Rheem 14, Takagi 941/991. Stop using hot water and call.
  5. Flow: Rheem P1: open the tap more. Noritz starts at 0.5 GPM.

One reset, only if your manual allows it

Navien front Reset button (p. 9); Rheem RTGH unplug about 30 s (p. 74; only a dry, undamaged cord, plug and outlet); Noritz code 11: power off at the remote, open a hot tap, power on (p. 42). Never flip a breaker to reset.

Makers’ minimum flows: Navien at least 0.5 gal/min (NPE-2 manual, p. 15); Noritz NCC199CDV starts at 0.5 GPM and can keep running down to 0.29 GPM (Owner’s Guide, pp. 39, 46); Stiebel Eltron 0.37–0.77 gal/min by model (p. 15); EcoSmart ECO 27 about 0.3 gal/min (p. 9); Rheem P1: increase water flow from the fixtures (RTGH Use & Care, p. 75). Checked October 15, 2026. Check your unit’s manual.

Open the hot tap more. Use the hot side only — a mixing valve set toward cold drops the hot flow the heater sees. On a Noritz, the Owner’s Guide (p. 39) discusses temperature swings and lists a temperature band that uses more hot water and less cold mix; check that page for the figure it shows, and feel the water at a tap before anyone uses it. Don’t raise the setting above what your household can use without scalding.

It’s not normal if

The same code returns after one reset; you smell gas or exhaust; the code is an overheat.

Is this normal?

A short cold burst between runs of hot water is common on a tankless heater — the burner pauses and restarts. Between back-to-back showers, leave a short gap or keep the hot tap running steadily instead of turning it off and on. If it keeps happening every time, use the cold-water sandwich steps in this guide.

Know exactly why not

Ignition codes that repeat need gas-pressure and flame-sensor testing inside the sealed combustion section. Fan, gas valve, sensor and board codes are parts behind the cover with live wiring. Scale codes need the water valves isolated and a descaling pump. Descaling means isolating the heater’s water valves and pumping cleaner through it; Rinnai and Rheem tell owners to call a pro (Rinnai LC p. 67; Rheem 1L p. 75).

We only list codes from the maker’s own manual. If yours isn’t in this list yet, tell your pro the exact code. Don’t clear it.

Start the guided check and choose Water heater, then a code on the display.

Sources

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Independent information, not an inspection or a confirmed diagnosis. Not attorney-reviewed and not a licensed HVAC review. support@secondwrench.co