Tankless water heater error codes are the machine’s own diagnostic report, and learning to read them turns a mystery lockout into a to-do item. Every brand numbers its codes differently, but the failures they describe are the same physics: ignition, flow, temperature, combustion air, and sensors. This reference maps the common codes by brand and, more usefully, sorts them into what you can fix and what a technician should see.
Read the pattern, not just the number
Before the tables, the diagnostic habit that matters: note when the code appears. A code at high demand but not low demand points at capacity or scale. A code on cold mornings points at combustion or venting. A code that follows a utility power flicker is often just a reset. And any code that recurs after clearing is a condition, not a glitch.
The scale family: codes that mean flush
Scale buildup in the heat exchanger announces itself through temperature and flow codes long before it destroys anything. This is the family where the fix is often a proper descaling rather than a part:
| Brand | Common scale-related codes | What the unit is seeing |
|---|---|---|
| Rinnai | LC, 10 | Scale accumulation warning; combustion airflow restricted |
| Navien | E407, E438, E515 | Outlet thermistor anomalies, abnormal operation |
| Rheem | 05, 99 | Imperfect combustion, blocked flue or exchanger side |
| Noritz | 10, 90, 920 | Air supply or exhaust restriction, combustion abnormality |
| Takagi | 101, 991 | Combustion abnormality warnings |
Two honest caveats. First, several of these codes are shared with venting problems, so check the intake and exhaust terminations for nests, lint, and snow before blaming scale. Second, a unit throwing these codes in hard water was overdue for maintenance before the display lit up; the earlier, quieter symptoms are listed in SIGNS YOUR TANKLESS NEEDS DESCALING. Rinnai’s LC code deserves special mention because it is explicitly a scale counter: the unit tracks combustion hours against expected exchanger condition and will eventually insist on service.
Ignition and flame codes
The most common lockouts on gas units: Rinnai 11, Navien E003 and E004, Rheem 11, Noritz 11 and 12, Takagi 111. The unit tried to light and failed, or lost flame after lighting. Homeowner-checkable causes, in order: gas supply valve partially closed, an empty propane tank, a tripped or loose power connection affecting the igniter, and blocked intake or exhaust. Past those, flame rods, igniters, and gas valves are technician territory, particularly on gas components.
Flow and temperature codes
Codes like Navien E012 (flow), Rinnai 16 (over-temperature), and Rheem 24 or 76 report water-side complaints. Over-temperature codes in particular are a classic scale signature, since scale insulates the exchanger and creates hot spots. Low-flow complaints often trace to the humble inlet filter screen, which strains debris where cold water enters and clogs quietly for years; cleaning it is a two-minute job described in the maintenance schedule. Crusted aerators at the tap fool units too, by keeping total flow under the activation threshold.
Sensor and board codes
Thermistor and sensor faults (Navien E407 family, Rinnai 32-33, various 7x codes across brands) sometimes mean a failed 20 dollar sensor and sometimes mean scale insulating a good sensor into reading nonsense. This ambiguity is another argument for keeping the exchanger clean: a descaled unit that still throws a sensor code has genuinely earned the part replacement. Flush kits from suppliers such as Chromex make the rule-out step cheap compared to a diagnostic visit.
When to stop and call
Gas-side repairs, repeated combustion codes after you have verified venting, any code involving the gas valve, and anything the manual marks as service-only. Also worth a call: a unit in warranty, because DIY repairs on gas components can void coverage that a documented service visit would preserve, a distinction covered in WATER HEATER WARRANTIES.
The bottom line
Error codes sort into five families: ignition, flow, temperature, combustion air, and sensors, and in hard water country an outsized share of them are scale wearing a different costume. Check venting, clean the inlet filter, flush on schedule, and keep your model’s manual bookmarked. The code is not the problem. It is the unit explaining the problem.