Tankless Water Heater Error Codes: What They Mean and How to Read Them
Reading Time: 6minutes
Tankless water heater error codes help homeowners quickly identify whether the issue is related to ignition, airflow, overheating, water flow, or a sensor failure. This guide explains how tankless water heater error codes work across major brands like Rinnai, Navien, Noritz, Takagi, and Rheem so you can understand the pattern behind each warning.
Why Tankless Water Heater Error Codes Show Up
Tankless water heater error codes confuse a lot of homeowners. One moment the system is working, and the next, the display flashes a number or a letter and the hot water shuts off. The reassuring news is that nearly all tankless water heater error codes fall into just a few categories. Once you recognize the pattern, the brand-specific numbering matters less, and the message becomes much easier to understand.
Every tankless unit uses these codes as a safety mechanism. If anything interrupts ignition, airflow, temperature control, water flow, or sensor communication, the control board steps in and stops the system. Think of the codes as checkpoints that alert you long before something serious develops.
Although the numbers vary, the meanings stay consistent across Rinnai, Navien, Noritz, Takagi, and Rheem.
Ignition Failures Show Up First
Ignition issues are one of the most common tankless water heater error codes. When your tankless unit tries to ignite, the flame either forms and stabilizes—or it doesn’t. If it fails to light or drops out immediately after forming, the unit sends an ignition code.
Rinnai tankless error codes 11 and 12 are normally related to this problem. Noritz tankless error codes use the same numbering. Navien labels ignition problems with E003 or E012. Takagi uses Code 111. Rheem mirrors Rinnai with Codes 11 and 12.
Most of these failures come from simple problems: a closed gas valve, a failing igniter, a dirty flame sensor, a blocked burner, or unstable air inside the combustion chamber. The display won’t fix the problem, but it does point in the right direction.
Proper installation also plays a major role. Small venting inconsistencies or borderline gas supply can trigger repeated ignition codes over time. Aaron Services installs tankless systems across North Atlanta with the setup and airflow they need to ignite reliably.
Venting and Air Supply Warnings Come Next
Tankless heaters rely on a consistent supply of clean air for combustion and a clear path to exhaust the burned gases. When something interferes with intake or exhaust, tankless water heater error codes related to venting appear.
Rinnai signals venting problems with Code 10. Noritz often uses Codes 90 or 99. Navien tankless error codes E010 or E030 indicate a venting issue, while Takagi uses Code 101. Rheem reports venting restrictions with Codes 29 or 61.
These warnings often point to blockages at the vent hood, debris at the termination point, loose vent joints, or clogged screens. Seasonal conditions matter too. In winter, ice can build up around exterior vents, while summer brings insects, leaves, and lint. Any of these can interrupt airflow and trigger a code.
Temperature and Overheating Codes Happen When Heat Spikes
Tankless systems use thermistors to monitor internal temperature. If the water heats too quickly or the heat exchanger rises above its safe limit, the unit shuts down and reports an overheating code.
Rinnai tankless error codes 14 and 16 are for temperature-related issues. Noritz uses Code 16. Navien tankless error codes flag these events with E002 or E016. Takagi uses Code 991. Rheem displays Code 13.
Overheating usually happens because of scale buildup inside the heat exchanger, restricted water flow, or aging temperature sensors. Scale causes hot spots, low flow increases internal temperatures, and worn sensors send incorrect readings. Older heat exchangers also tend to run hotter as they age.
If you’re unsure how long your system should last, our guide on the life expectancy of a water heater covers normal tankless lifespan trends.
Water Flow Codes Point to Supply-Side Problems
Tankless water heater error codes tied to water flow show up when the system doesn’t detect enough movement through the unit. Tankless heaters need a minimum flow rate to activate; if the flow stays below that threshold, the heater won’t fire.
Rinnai uses Codes 18, 31, and 65. Noritz uses Codes 42, 61, or 66. Navien tankless error codes E011 or E039 relate to this water flow issue. Takagi uses Codes 721 and 731. Rheem displays P1 and P2.
Most flow-related codes trace back to clogged inlet filters, partially closed valves, dips in municipal water pressure, or mineral buildup inside fixtures. Flow sensors also age and drift over time.
In some cases, you won’t see a code at all. If a faucet only dribbles, the flow may be too low for the heater to acknowledge. The system stays silent, never fires, and simply gives you cold water.
Sensor Faults and Internal Failures
Modern tankless heaters rely on internal electronics—control boards, fans, wiring harnesses, pressure sensors, and thermistors—to run properly. When one of these components fails or loses communication, the unit reports an internal fault.
Rinnai tankless error codes 70 through 75 refer to sensor faults and internal failures. Noritz uses 73 or 760. Navien tankless error codes show E007, E008, or E036. Takagi flags this as Code 741. Rheem uses Codes 76 or 79.
These codes usually point to electrical wear, moisture near the control board, loose connectors, or aging sensors. Internal fault codes confirm the problem is inside the heater—not with the gas or water supply. These are service-level repairs that require accessing sealed panels. Aaron Services repairs tankless systems weekly and handles everything from faulty boards to drifting sensors.
Common Tankless Water Heater Error Codes Reference Chart
Even though tankless water heater error codes vary from brand to brand, the message stays consistent. Because we’ve serviced so many tankless units, we’ve been able to identify the most common tankless water heater error codes. Everything fits into five categories: ignition, airflow, overheating, flow, and electronics. Once you know the category, you know the general cause.
Here’s an easy reference chart:
Error Category
Rinnai tankless error codes
Noritz tankless error codes
Navien tankless error codes
Takagi tankless error codes
Rheem tankless error codes
Ignition / Flame
11, 12
11, 12
E003, E012
111
11, 12
Air Supply / Venting
10
90, 99
E010, E030
101
29, 61
Overheat / Temperature
14, 16
16
E002, E016
991
13
Water Flow / Pressure
18, 31, 65
42, 61, 66
E011, E039
721, 731
P1, P2
Sensors / Internal Fault
70–75
73, 760
E007, E008, E036
741
76, 79
Literature and Guides from Leading Tankless Brands
Not all tankless water heater problems create an error code. Sometimes a faucet is barely open, keeping flow too low for the heater to recognize. Clogged aerators or fixture restrictions can cause the same situation. The unit thinks no one needs hot water, so it doesn’t fire and never alerts you.
Another common tankless water heater problem customers call us about is often called a “cold water sandwich.” This term refers to an event where hot water, normally during a shower, momentarily turns cold, and then returns back to hot water again.
These situations can be especially frustrating because the there’s no indication of any tankless water heater error codes despite the problems users experience.
What Homeowners Should Do With Tankless Water Heater Error Codes
When tankless water heater error codes appear, the goal is to find out whether it’s a simple fix or something that needs service.
Homeowners can check the basics: • Gas supply open • Hot and cold water valves fully open • Power restored • Exterior vents clear • Inlet filter cleaned if accessible • Faucets opened fully to test demand
Anything beyond that becomes a technician-level repair. Tankless systems have sealed combustion areas, internal wiring, electronic boards, and components that require diagnostic tools.
Aaron Services resolves tankless water heater error codes every day throughout North Atlanta. Our technicians troubleshoot ignition issues, restore airflow, descale heat exchangers, correct flow restrictions, and repair internal components. Once the root cause is resolved, the hot water returns and the system runs normally again.
Why does my tankless water heater stop working without showing an error code?
The minimum flow rate may not be met. A slow faucet stream or a clogged aerator keeps the heater from detecting demand.
What do tankless water heater error codes usually mean?
The codes point to ignition issues, vent problems, overheating, low water flow, or sensor faults. Each brand uses different numbers, but the categories stay consistent.
Do I need a plumber to fix tankless water heater error codes?
The codes help narrow down the category, but the internal repairs often require training. Gas, venting, and electronics need proper service.
Are tankless water heater ignition codes dangerous?
The unit shuts itself down before any risk develops. The code signals that ignition didn’t complete as expected.
How often should I maintain my tankless water heater?
