Quick Answer: Tankless error codes fall into two families that don’t share a naming scheme: Rinnai and Rheem both use bare numeric codes and — coincidentally — assign the same numbers to the same faults (11 = No Ignition, 12 = Flame Failure, 14 = Thermal Fuse/Overheat, 16 = High Outlet Temperature, per both manufacturers’ own documentation), while Navien uses an entirely separate E-prefix scheme (E003, E004, E016…). Some faults are owner-fixable in minutes — a closed gas valve, a snow-blocked vent, a dirty aerator screen — while anything touching the internal gas valve, PC board, or igniter assembly needs a licensed technician, and Rinnai’s own manual warns that adjusting the internal gas valve voids the warranty outright.
A blinking code on the temperature controller is the single most common reason a tankless owner searches for help, and the advice online is a mess of contradictory forum posts. This guide sticks to what the three manufacturers whose lineups we cover most — Rinnai, Navien, and Rheem — actually publish in their own troubleshooting documentation, decoded side by side.
Cross-brand code decoder
| Fault | Rinnai code | Rheem code | Navien code | DIY-safe first check? |
|---|---|---|---|---|
| No ignition | 11 | 11 | E003 | Yes — verify gas valve open, correct gas type |
| Flame failure / loss | 12 | 12 | E012 | Partial — check for visible flame in view window |
| False flame detection | — | — | E004 | No — grounding/igniter issue, call a tech |
| Thermal fuse / overheat (ODS) | 14 | 14 | E016 | No — safety trip, needs root-cause diagnosis |
| High outlet water temperature | 16 | 16 | — | Yes — check vent restrictions, fan operation |
| Low water flow | — | P1 | — | Yes — clean aerators, check inlet filter |
| Scale buildup | LC / LC0–LC9 | IL | E030, E760 | Yes — see our descaling guide |
| Water leak detected | 79 | — | — | Partial — inspect cabinet for pooling first |
| Freeze protection (no burner) | 02 | — | — | Yes — verify power stays on to the unit |
| Exhaust blockage | — | — | E110 (3 sub-codes) | Yes — clear vent terminal, check fan |
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By the numbers
- Rinnai’s Code 16 high-outlet-temperature shutdown trips at exactly 203°F (95°C) — a hard safety threshold documented in Rinnai’s own “Troubleshooting Tankless Water Heater Diagnostic Codes” manual (document 100000652), not an approximate figure.
- The spark electrode gap behind a Code 11 no-ignition fault must measure 0.138 in. (3.5 mm) on Rinnai units — too wide or too narrow and the igniter can’t reliably arc, per the same Rinnai diagnostic manual’s Code 11 troubleshooting steps.
- Rheem’s P1 low-flow code trips when measured flow at the unit drops below 0.66 GPM for more than 5 seconds — below the minimum needed to trigger main burner ignition, per Rheem’s documented fault-code guidance (cross-verified across multiple independent service-technician references, since Rheem does not host a single public error-code page).
Reading Rinnai’s numeric codes
Rinnai groups its lineup into two families — non-SENSEI (the older V/RL/KB/KCM series) and SENSEI (RU/RUR/CU) — and most codes apply to both, per its own diagnostic manual.
- Code 02 — No Burner Operation During Freeze Protection (non-SENSEI only). Not a malfunction on its own; check that gas valves are open and gas type is correct before assuming a fault.
- Code 11 — No Ignition (both series). The unit attempted to fire and failed. Rinnai’s troubleshooting starts with confirming all gas valves are open, inlet gas pressure is within spec on the rating plate, and — critically — that the gas type DIP switch or Parameter 10 setting (A = natural gas, B = propane) matches the actual supply.
- Code 12 — Flame Failure (both series). The burner lit, then lost flame. Rinnai’s own steps point first to the flame rod and its silicone protective sleeve, then to vent-terminal clearances, since improper clearance can cause combustion-gas recirculation that leads directly to a Code 12.
- Code 13 — Combustion (non-SENSEI only). Points at venting materials, combustion air supply, and altitude DIP switch settings rather than the burner itself.
- Code 14 — Thermal Fuse or Overheat Sensor (ODS) (both series). Rinnai’s manual calls this “an indication of overheating” that “should be taken seriously” — the bi-metal switch resets automatically once the heat exchanger cools but keeps tripping if the underlying cause isn’t fixed.
- Code 16 — High Outgoing Water Temperature, a safety shutdown once outlet water exceeds 203°F (95°C) (both series). Rinnai’s first checks are vent restrictions, fan operation, and heat-exchanger blockage.
- Code 79 — Water Leakage Detected (non-SENSEI only). Rinnai’s steps start with the leak-sensor wiring harness, then a visual check of the cabinet for pooling before assuming the sensor itself is defective.
- LC / LC0–LC9 and SS are scale and service-reminder indicators rather than hard faults — covered in full, including the five-button-press workaround, in our tankless water heater descaling guide.
Reading Navien’s E-codes
Navien’s NPE service manual uses an entirely different scheme: a three-digit number after the letter E, sometimes with a sub-code. From the manual’s own error-code table:
- E003 — Ignition failure.
- E004 — False flame detection, which points at a loose ground wire or an igniter not producing a spark rather than an actual fire risk.
- E012 — Flame loss during normal operation.
- E016 — Overheating of heat exchanger.
- E030 — Exhaust overheat, tripping above 230°F (110°C), with sub-codes for a shorted or open exhaust thermistor.
- E109 — Abnormal fan motor activity.
- E110 — Exhaust blockage, split into three sub-codes depending on whether the blockage shows up during fan-only operation, an active hot-water draw, or space-heating mode.
Navien’s own Lime Alarm (E760) and the scale-related E016/E030 pairing get separate, deeper treatment in the descaling guide linked above, since those two codes double as both hard faults and maintenance flags.
Reading Rheem’s codes
Rheem’s tankless lineup — spanning the retail Performance/Performance Plus/Platinum line and the pro RTGH series — publishes its own numeric and letter codes, several of which line up exactly with Rinnai’s numbering:
- Code 00 — Continuous combustion for more than one hour, often a stuck recirculation pump or a blocked vent system rather than a burner fault.
- Code 05 — Combustion problem.
- Code 11 — No ignition (identical meaning to Rinnai’s Code 11).
- Code 12 — Flame failure (identical meaning to Rinnai’s Code 12).
- Code 14 — Overheating (identical meaning to Rinnai’s Code 14).
- Code 16 — Outlet temperature too high, typically from a clogged heat exchanger or restricted vent terminal (identical meaning to Rinnai’s Code 16).
- Code 24 — Operational switch fault.
- Code 29 — Heat exchanger outlet temperature too low.
- Code 71 — Gas inlet solenoid valve fault.
- Code P1 — Low water flow, below 0.66 GPM for more than 5 seconds — insufficient to sustain main burner ignition.
- Code IL — Scale buildup indicator in the heat exchanger, Rheem’s equivalent of Rinnai’s LC family.
What’s safe to check yourself before calling a pro
- Power cycle the unit — turn off, wait 60 seconds, restart. Resolves a meaningful share of one-off sensor glitches without touching gas or wiring.
- Confirm the gas valve is fully open and, on propane systems, that the tank actually has fuel — the single most common root cause behind a Code 11 / E003 service call.
- Check the gas-type setting matches your actual supply (natural gas vs. propane) on the rating plate, DIP switch, or Parameter 10 setting.
- Clear the vent terminal of snow, leaves, nests, or other debris — a leading cause of Code 16, Rheem Code 00/16, and Navien E110.
- Clean aerator screens and the inlet water filter — the fix behind most Rheem P1 low-flow faults and a fair share of intermittent shutdowns generally.
When to stop and call a licensed technician
Rinnai’s safety instructions are unambiguous on this point: “DO NOT adjust the internal gas valve. The design is such that adjustment is not required. Warranty will be voided if the internal gas valve is adjusted.” That single line covers most of what shouldn’t be DIY territory — along with igniter and flame-rod replacement (which requires disconnecting live components and correct 0.138 in. electrode gap re-verification), PC board diagnostics, and anything requiring a manometer on the gas line. Codes 12, 14, and 71/E016 in particular tend to trace back to one of these, and Rinnai’s own manual gates several of its later troubleshooting steps behind “you must be qualified to service gas systems.”
The bottom line
Rinnai and Rheem share the same numbers for the same faults (11, 12, 14, 16) even though the two companies publish separate manuals — a genuine convenience if you’ve owned one brand and switched to the other. Navien breaks from that pattern entirely with its E-prefix scheme. Either way, start with the owner-safe checks — gas valve, gas type, vent clearance, aerator screens — before assuming a part has failed, and treat anything involving the internal gas valve or igniter assembly as licensed-technician territory, exactly as Rinnai’s own manual instructs.
If your code turns out to be scale-related (Rinnai LC, Rheem IL, Navien E030/E760), our descaling guide covers the fix in full. For the other three recurring maintenance items that prevent most of these codes from happening in the first place, see our full maintenance checklist, and if a code is telling you the unit itself needs replacing rather than repairing, our best tankless water heater picks is the place to start.