Short answer
Most problems follow the type, not the brand: tanks fail in the burner, elements or tank, and tankless units flag faults with codes. The brand tells you what each code means and how to read the build date. Homeowners can check power, gas, valves and filters; sensors, gas parts and wiring need a licensed plumber.
Key facts
- Navien's NPE-2 manual assigns the hot water outlet sensor to code E407 and the main circuit board to E515, although several websites mix them up.[3]
- Rinnai's LC code warns of scale building up in the heat exchanger, and it is the one Rinnai code that lets the unit keep running.[2]
- HomeGuide's national average for a tank water heater repair is $455, with most between $150 and $700 (August 2025).[5]
- Rheem says that if a repair costs more than 50% of a new unit, replacement is almost always the better investment.[1]
Safety first
If you smell gas near the heater, do not troubleshoot. Leave the building, do not use switches or phones inside, and call the gas utility or 911 from outside[9]. Also stop and call a licensed plumber if the relief valve is blowing hot water or steam, or if water is coming from the tank body.
Pick your brand
| Brand guide | Units covered | Where to start |
|---|---|---|
| Navien | Condensing tankless and combi units | Read the E-code. E407 is the outlet sensor and E515 the main board, per Navien's manual[3] |
| Rinnai | Tankless, condensing and non-condensing | Codes 10 and 25 point to venting or a full condensate trap; LC is scale[2] |
| Rheem tankless | Gas tankless, indoor and outdoor | 1L is scale; 10, 90 and 99 involve venting or the blower[4] |
| Takagi | Gas tankless | Read the code, then check gas, water valves and the inlet filter |
| Noritz | Gas tankless | Read the code, then check gas, water valves and the inlet filter |
| A.O. Smith | Tanks, tankless, heat pump | A.O. Smith names hard water deposits as the main cause of tankless failure[6] |
| Bradford White | Gas and electric tanks | Removing the anode voids the tank warranty, per Bradford White[7] |
| Rheem tank | Gas, electric and hybrid tanks | The first four serial digits give the build month and year[8] |
For a full list of codes by brand, start at the error code hub.
What a homeowner can safely check
These need no tools and no opened panels:
- Gas valve on and gas service working; breaker not tripped; tankless unit plugged in.
- Cold water supply valve fully open.
- Any error code written down before a reset.
- Outside vent terminal clear of snow, leaves or nests.
- Condensate line draining and not frozen (condensing units).
- The cold water inlet filter on tankless units, cleaned per the owner's manual. Navien also has an intake air filter[3].
One reset is a fair test. If the code returns, stop. Rinnai tells owners not to keep resetting its overheat code[2].
What needs a licensed plumber
Gas valves, burners, pilot assemblies, igniters and flame sensors; heating elements, thermostats and wiring; sensors, fans and circuit boards inside tankless cabinets; relief valve replacement; venting changes; and any leak from the tank body. A leaking tank cannot be repaired.
Typical repair costs
HomeGuide's national figures (August 2025)[5]:
| Repair | Typical range |
|---|---|
| Tank repair, average | $150 to $700 |
| Thermocouple | $100 to $325 |
| Gas control valve | $150 to $550 |
| Heating element | $100 to $350 |
| Igniter | $75 to $350 |
| Anode rod | $150 to $350 |
| Tankless repair | $300 to $1,300 |
| Tankless flush and descale | $75 to $300 |
Service call fees of $50 to $200 may apply[5]. If a quote tops 50% of a new unit's price, Rheem's guidance is to replace instead[1]. See repair or replace and check the unit's age with our age decoder.
Every brand repair guide
- Navien water heater repair
- Rinnai water heater repair
- Rheem tankless water heater repair
- Takagi water heater repair
- Noritz water heater repair
- A.O. Smith water heater repair
- Bradford White water heater repair
- Rheem tank water heater repair
When to call a pro
- Any gas smell, scorch marks, or hot water or steam from the relief valve.
- A fault code that returns after one reset.
- Any work on gas parts, wiring, sensors, boards or venting.
- Water leaking from the tank itself, or a leak you cannot trace.
More questions
Will a repair void my warranty?
It can. Many makers require a licensed installer or servicer and genuine parts, and some deny claims on units that were modified. Bradford White, for example, says removing the anode voids its tank warranty. Read the warranty before anyone opens the unit.
Sources
- Water heater lifespan: when to repair vs. replace (Mar 25, 2026), Rheem Manufacturing. Accessed 2026-10-06.
- Troubleshooting Tankless Water Heater Diagnostic Codes (document 100000652), Rinnai America. Accessed 2026-10-06.
- NPE-2 User's Information Manual (2020-12), Navien. Accessed 2026-10-06.
- Tankless Gas Troubleshooting Pocket Manual SVC820, Rheem Manufacturing. Accessed 2026-10-06.
- Hot water heater repair cost (Aug 7, 2025), HomeGuide. Accessed 2026-10-06.
- How long do tankless water heaters last?, A.O. Smith. Accessed 2026-10-06.
- Hydrogen sulfide odor (rotten egg smell) and chlorinating water heaters, Bradford White. Accessed 2026-10-06.
- How to locate and read your Rheem water heating serial numbers, Rheem Manufacturing. Accessed 2026-10-06.
- What to do if you smell natural gas, CenterPoint Energy. Accessed 2026-10-06.
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