Tankless Water Heater Error Codes: What They Mean and How to Fix Them

A flashing error code on your tankless water heater display is the unit telling you exactly what is wrong. Most codes point to specific components and some can be resolved with a simple reset. Others require a licensed plumber or HVAC technician. This guide covers the most common error codes for Rheem, Navien, and Rinnai units: the brands we service most frequently in Tampa Bay.

General Rules for Tankless Error Codes

Before looking up your specific code, keep these points in mind:

• Write down the exact code. Error codes on the same brand can have very different meanings: Error 11 and Error 12 on a Navien are completely different problems. Note the exact number or letter combination before resetting.
• Try a manual reset first. Many codes will clear after shutting off the unit for 30 seconds and restarting. If the code comes back within a few minutes or hours, the underlying problem needs service.
• Check for simple causes first. Gas supply valve off, low water pressure, or a closed cold-water supply valve causes many error conditions that show up as unit codes.
• Do not ignore persistent codes. Some codes indicate a safety shutdown. Continuing to use the unit while ignoring a persistent safety error can damage the unit or create a safety hazard.

Rheem Tankless Water Heater Error Codes

Rheem uses a lettered code system on their RTGH series and similar units. We install Rheem tankless systems across Tampa Bay and see these codes regularly.

Navien Tankless Water Heater Error Codes

Navien units use a numbered error system. Navien NPE series units are common in newer Tampa Bay construction.

Rinnai Tankless Water Heater Error Codes

Rinnai units use a two or three digit numeric system. Rinnai RU and RL series units appear frequently in Tampa Bay homes and condos.

Tampa Bay-Specific Error Causes

Several conditions common in our area drive specific error codes more than the national average:

• Scale buildup (codes related to overheating). Tampa Bay’s hard water (the City of Tampa averaged 10.8 grains per gallon in 2024) deposits calcium inside the heat exchanger faster than in softer water regions. Heat exchangers in homes without water softeners or conditioners can develop significant scale within 3 to 5 years. Regular flushing with white vinegar solution or descaling service is the fix.
• Blocked vent termination from pests. Tankless unit intake and exhaust pipes typically terminate on an exterior wall or roof. Insects and small animals in Tampa Bay (particularly paper wasps and mud daubers) can build nests at vent terminations, triggering combustion air and exhaust error codes.
• Low water pressure errors. High demand from irrigation systems, multiple simultaneous uses, or aging supply pipes can drop water pressure below the unit’s minimum flow rate, causing flow or heating errors.

When to Call for Professional Service

Call Home Therapist when:

• The error code returns within a few cycles after resetting
• The code relates to ignition, gas, combustion, or venting
• You see error codes related to overheating or thermal fuse trips
• The unit is more than 5 years old and has never been descaled
• Any code that says professional service is required in the table above

Related: Why Is My Water Heater Leaking?

Frequently Asked Questions

• What are the common error codes for tankless water heaters?

  The most common tankless water heater error codes fall into four categories: ignition failure (e.g., Rheem C2, Navien E003, Rinnai 11), overheating or high-limit trip (e.g., Navien E047, Rinnai 14), exhaust or venting blockage (e.g., Rinnai 16, Navien E010), and water flow too low to fire the burner. Each brand uses its own numbering, so always match the code to your model's manual before attempting a reset.

• Why is my water cold with a tankless water heater?

  Cold water from a tankless unit almost always means the burner never ignited. The most common causes are an ignition failure code (no gas, weak igniter, or dirty flame sensor), a flow sensor that does not detect enough demand to trigger heating, or a tripped high-limit that locked the unit out. Check the display for an error code first; that code identifies the exact subsystem that failed.

• How do I reset a tankless water heater?

  Press and hold the power button for three to five seconds, or turn the unit off at the breaker for 30 seconds, then restore power. If the same error code returns immediately, the underlying fault has not cleared and a licensed plumber should diagnose it. Home Therapist provides FREE diagnosis on every visit, so you pay nothing to find out what the code actually means.

• What are the signs of a tankless water heater going bad?

  Warning signs include recurring error codes that reset but return within days, fluctuating water temperature during a single shower, visible scale buildup or reduced flow at hot-water fixtures, a burner that ignites then shuts off mid-use, and error codes related to heat-exchanger overheating. In Tampa Bay's hard-water and high-humidity climate, scale accumulation inside the heat exchanger is a leading cause of premature failure. Home Therapist has 1,300+ five-star reviews diagnosing and repairing tankless units across the region.

• What are the venting requirements for Rinnai tankless water heaters?

  Venting depends on the model: condensing Rinnai units use plastic vent pipe, while non-condensing units need stainless steel vent, and each has set vent length and clearance limits in its installation manual. We service all brands, and our FREE diagnosis checks the vent for blockage, damage or poor termination.