Tankless Water Heater Error Codes Explained

You will see tankless water heater error codes flash on the digital monitor whenever a modern unit detects a fault.

We know how stressful an unexpected cold shower can be for a homeowner or business manager. These opaque codes actually point directly to the source of the problem.

Most error numbers translate into one of seven clear failure categories.

Our team put together this brand-by-brand reference to decode the alerts you will most likely encounter. Understanding these signals is the first step toward restoring your hot water.

Code Categories Across All Brands

Most tankless error codes map to ignition, gas supply, water flow, combustion, heat exchanger, control board, or overheating issues. Fixing these problems promptly is crucial because a well-maintained tankless system provides exceptional long-term value.

According to the US Department of Energy in 2026, tankless water heaters are up to 34% more efficient than traditional storage models. They also boast a lifespan of over 20 years. Standard tanks typically fail after just 10 to 15 years.

We frequently see these seven main categories during service calls:

• Ignition or flame loss (most common)
• Gas supply issues (low pressure, closed valve)
• Flow and water side (clogged filter, low pressure)
• Combustion and venting (blocked vent pipe, draft problems)
• Heat exchanger faults (scale buildup, overheating)
• Control board sensors (electrical short or fault)
• Urgent overheating (urgent, stop using)

Rinnai Error Codes

Rinnai error codes frequently point to ignition failures, flame loss, or blocked venting. Codes 11 and 12 are the most common alerts you will see on a Rinnai digital monitor.

We handle Code 11 calls constantly. A frequent fix for new US installations is simply bleeding trapped air from the gas lines. For older setups, contractors often need to replace the igniter coil or inspect the gas meter for proper inlet pressure.

Rinnai Code 12 indicates the PC board detected a flame momentarily before losing it. The flame rod measures flame intensity in micro-amps. A quick cleaning of this yellow wire often restores the signal.

Code	Category	Likely Cause	Action
10	Combustion air	Blocked intake or vent	Inspect vent termination
11	Ignition	No gas or ignition failure	Bleed gas line, check igniter
12	Flame loss	Flame detected then lost	Clean flame rod sensor
14	Overheat	Heat exchanger over-temperature	Stop use, call urgently
16	Hi-limit	Output temp above safety threshold	Call for diagnostic
LC	Scale	Heavy scale buildup detected	Schedule descaling

Navien Error Codes

Navien error codes heavily feature ignition faults like E003 and flow sensor issues like E012. These alerts help pinpoint exactly which component requires cleaning or replacement.

Our technicians often clear an E012 flow sensor code by opening and closing the pressure relief valve near the outlet to reset the system. Older models sometimes suffer from corroded sensors. Upgrading to a corrosion-free stainless steel version usually prevents the E012 error from returning.

Navien E003 indicates an ignition failure. This typically traces back to a malfunctioning gas valve assembly or a dirty dual venturi system that needs professional attention.

Code	Category	Likely Cause	Action
E001	Ignition	No ignition	Check gas supply
E003	Ignition failure	Gas valve or venturi issue	Call for diagnostic
E007	Overheat	Heat exchanger over-temp	Stop use, call urgently
E012	Flow sensor	Flow sensor fault or exhaust block	Clean filter, upgrade sensor
E016	Air	Intake/exhaust restriction	Inspect vent
E760	Water flow	Low water flow	Check inlet filter

Noritz Error Codes

Noritz units commonly display Error Code 90 for combustion abnormalities, alongside standard ignition alerts like Code 11 and 12. These alerts mean the system is struggling to reach the set temperature.

Code 90 is a major pain point for Noritz owners. On new installations, this usually means the gas line or meter is undersized and fails to provide minimum inlet pressure during a stress test.

We also see Code 90 on older units. In these cases, a clogged condensate drain line under the heater or debris buildup at the vent terminations is the usual culprit.

Code	Category	Likely Cause	Action
11	Ignition	No ignition	Check gas, power-cycle
12	Flame loss	Flame lost during operation	Clean flame sensor
14	Overheat	Heat exchanger over-temp	Stop use, call urgently
16	Hi-limit	Output temperature too high	Call for service
51	Combustion	Combustion air issue	Inspect vent and intake
90	Combustion	Airflow or combustion abnormality	Check gas size, clear drain

Rheem Error Codes

Rheem water heaters use alpha-numeric codes like C1 for ignition failure and numeric codes like 76 for communication defects. Resolving these requires checking both mechanical components and electrical connections.

Our service team uses a great trick to diagnose Error Code 76. This code indicates a communication breakdown between the main circuit board and the remote control. You can isolate the problem by disconnecting the remote wires from the board and turning on a hot tap.

The main burner will fire if the circuit board is working perfectly. This tells you the remote control or its wall wiring needs replacement. For Rheem Code 11, checking for blocked venting or replacing the igniter coil usually solves the ignition failure.

Code	Category	Likely Cause	Action
C1	Ignition	No ignition	Check gas, power-cycle
C2	Flame loss	Flame lost	Call for service
C7	Combustion fan	Fan motor fault	Call for diagnostic
C8	Combustion air	Vent restriction	Inspect vent
11	Ignition error	Ignition failure	Replace igniter coil
76	Communication	Remote control defect	Test board, check wiring

How to Self-Diagnose Before Calling

You can safely perform basic self-diagnostics by noting the exact code and resetting the system power. A simple reboot often clears temporary sensor glitches.

We recommend a careful approach to avoid voiding your warranty or damaging the unit. Most professional diagnostics take between one and two hours to complete thoroughly.

Follow these steps to check your system:

1. Note the exact code (some brands show it briefly)
2. Power-cycle the unit (breaker off 30 seconds)
3. Wait 5 minutes and try operation again
4. If the code returns, call for diagnostic
5. If an overheat code appears, stop using immediately and call urgently

Annual maintenance is the best defense against these faults. Flushing the system and performing a descaling treatment every 12 to 18 months prevents the scale buildup that triggers flow and heat exchanger errors.

When You Need a Technician Now

Certain error categories require immediate professional intervention to ensure your safety and protect the equipment. Overheating and gas-related faults should never be ignored.

Our local experts are ready to handle these critical situations. Three specific error categories warrant same-day calls:

• Overheat codes (Rinnai 14, Navien E007, Noritz 14, Rheem variants), stop using
• Combustion-related codes with any gas smell, shut off the gas valve and call
• Recurring ignition failures with audible gas flow, call before continued attempts

For routine tankless water heater error codes that need diagnostic but are not urgent, our tankless repair service gets you a vetted local technician with OEM parts. Most repairs land same-day or next-day.

For more on no-heat troubleshooting before calling, see our tankless not heating guide.