A tankless water heater error code is your system’s way of identifying a specific problem. Some codes point to simple issues you can check yourself, while others indicate gas, venting, electrical, or internal component problems that need professional service. Understanding what the warning means helps you respond appropriately instead of relying on trial and error.

Modern tankless water heaters constantly monitor water flow, burner operation, exhaust, and temperature. If something falls outside normal operating conditions, the unit displays an error code to help narrow down the cause. Arizona homeowners often place heavy demands on these systems, and hard water can also contribute to recurring warnings over time. At Desert Drain Plumbing, we answer many questions about system diagnostics during water heater installation in Tempe, AZ, making it helpful to understand what these codes are trying to communicate before hot water suddenly disappears.

Why Your Tankless Water Heater Shows Error Codes

Unlike conventional storage tanks, tankless systems use electronic controls and sensors to monitor nearly every part of the heating process. Instead of continuing to operate with a potential problem, the unit often shuts itself down and displays an error code.

That warning is not always a sign that something has broken. In many situations, the system is preventing unsafe operation until the underlying issue is corrected.

Common Tankless Water Heater Error Codes and What They Mean

The exact number on your display depends on the manufacturer. Rinnai, Navien, Noritz, Rheem, and Bosch all use different numbering systems, but many codes point to the same underlying issue.

Ignition Failure

Common error codes:

• Rinnai: 11
• Navien: E003
• Noritz: 11
• Rheem: 11 (many models)

What it means:

The burner could not ignite because the unit isn’t getting the right conditions to start heating water.

Possible causes:

• Gas supply interruption
• Faulty igniter
• Dirty flame rod
• Air in the gas line

What to do:

Confirm the gas valve is open and try one manufacturer-approved reset. If the code returns after those checks, the ignition system, gas supply, or flame sensor may need professional diagnosis. At that point, tankless water heater repair is typically the appropriate solution.

Flame Failure

Common error codes:

• Rinnai: 12
• Navien: E012
• Noritz: 12
• Rheem: 12 (many models)

What it means:

The burner ignited but the flame went out during operation.

Possible causes:

• Low gas pressure
• Dirty flame sensor
• Venting issue
• Internal component fault

What to do:

Repeated flame failures should be diagnosed by a plumber.

Overheating

Common error codes:

• Rinnai: 16
• Navien: E016
• Noritz: 16
• Rheem: 16 (many models)

What it means:

Water or internal components became hotter than the unit allows.

Possible causes:

• Scale buildup
• Restricted water flow
• Faulty temperature sensor

What to do:

Turn the unit off and arrange service if the code returns.

Air Intake or Venting Problems

Common error codes:

• Rinnai: 10
• Navien: E110
• Noritz: 90 or 99
• Bosch: A3 (selected models)

What it means:

The heater isn’t receiving enough combustion air or can’t vent exhaust properly.

Possible causes:

• Blocked vent
• Debris
• Snow, nests, or leaves near the termination
• Fan malfunction
• Temperature Sensor Problems

Temperature Sensor Problems 

Common error codes:

Rinnai: 32, 33, 34, 35, 36, 38

Navien: E046

Noritz: 31

Rheem: Varies by model

What it means:

One of the temperature sensors is reporting abnormal readings.

Fan Motor Problems

Common error codes:

• Rinnai: 61
• Navien: E110 (can also indicate airflow-related fan issues)
• Noritz: 59
• Rheem: Varies by model

What it means:

The combustion fan isn’t operating correctly.

Water Flow Problems

Common error codes:

• Rinnai: 65
• Navien: E110/E351 depending on model and cause
• Noritz: 63
• Rheem: Varies by model

What it means:

The unit isn’t detecting proper water flow through the heat exchanger.

Possible causes:

• Dirty inlet filter
• Mineral buildup
• Flow sensor issue

What You Can Check Before Calling

Not every warning requires immediate repairs. Before scheduling service, you can safely check a few basic items:

• Verify the unit has electrical power.
• Confirm the gas valve is fully open.
• Check for obvious debris around exterior vent openings.
• Clean the inlet water filter if permitted by the manufacturer.
• Perform a single reset using the instructions in your owner’s manual.

Avoid removing sealed panels or working on gas or electrical components yourself.

When Professional Service Makes Sense

A code that appears once and never returns may have resulted from a temporary interruption. Recurring warnings are different. When the same code continues appearing or multiple codes develop over time, the underlying issue needs a complete diagnosis.

In some situations, an aging unit with repeated component failures may make tank or tankless water heater replacement the more practical investment than continuing to repair older equipment. A plumbing inspection can help determine which option offers the best long-term value.

Frequently Asked Questions About Tankless Water Heater

Can I keep using my tankless water heater after an error code disappears?

If the warning clears and does not return, the interruption may have been temporary. If the same code comes back, schedule a professional inspection.

Do all tankless water heaters use the same error codes?

No. Every manufacturer has its own numbering system, although many codes represent similar operating conditions.

Does Arizona’s hard water cause error codes?

Yes. Mineral deposits can restrict water flow, reduce heat transfer, and trigger overheating or maintenance-related warnings if the heat exchanger is not flushed periodically.

Should I keep resetting the unit until it works?

No. Repeated resets do not fix the underlying problem and can delay repairs while the issue continues.

Don’t Ignore What Your Water Heater Is Trying to Tell You

An error code is more than a flashing number. It is your water heater’s way of identifying a condition that needs attention before reliable hot water is interrupted. Desert Drain Plumbing provides fast same-day service, 24/7 emergency plumbing service for urgent situations, free in-person quotes, and financing for qualifying projects. As a family-owned and operated plumbing company, we help homeowners diagnose tankless water heater problems accurately and recommend practical solutions. 

If your unit keeps displaying error codes, contact our team so we can identify the cause and restore dependable hot water.