Tankless water heater troubleshooting starts with the symptom pattern. The source material here covers error codes, flame loss, scale, minimum activation flow, and the cold-water sandwich effect. It does not identify every possible failure, so the page stays focused on those verified problem patterns.
Use Error Codes as Clues
Per an industry/contractor source, ignition failure can appear as Rinnai code 11, Rheem 11, or Navien E003. The same type of source says flame failure or flame loss can appear as Rinnai code 12, Rheem 12, or Navien E012. Those codes should be read as clues, not as a complete diagnosis by themselves.
The important distinction is that ignition failure and flame loss are not the same symptom. Ignition failure points to the burner not lighting successfully. Flame loss points to a flame that is not staying established. That difference affects how a service conversation should begin, even though the source does not identify every underlying cause.
A homeowner can use the displayed code to describe the problem accurately. Instead of saying the unit is broken, report whether the displayed pattern looks like ignition failure or flame loss. That keeps the conversation tied to the code list and avoids turning a code into an unsupported parts recommendation.
The same discipline applies when comparing advice from different places. A code can name a failure pattern, but it should not be stretched into a guaranteed cause. Treat the display as a starting point for diagnosis, then keep the follow-up questions tied to the symptom actually shown.
Separate Ignition Failure From Flame Loss
The industry/contractor source groups code 11 patterns with ignition failure and code 12 patterns with flame failure or loss. That split is useful because it keeps troubleshooting language precise. A unit that cannot ignite and a unit that loses flame after ignition may need different review, even when both problems interrupt hot water.
The source does not verify a universal fix for either code family. It only describes how those code patterns appear across Rinnai, Rheem, and Navien examples. A careful troubleshooting process should therefore avoid promising that cleaning, resetting, or replacing a specific part will resolve every code.
For practical use, write down the brand name, the exact code shown, and the hot-water behavior around the failure. That record gives the technician a cleaner starting point. It also prevents a common mistake: treating every tankless shutdown as the same problem.
Scale Can Contribute to Flame Problems
Per an industry/contractor source, scale on the heat exchanger, flow sensor, or temperature sensor can cause flame loss. That fact connects a common maintenance issue with a shutdown symptom. It also explains why scale should not be treated as only an efficiency issue.
The phrasing matters. The source identifies scale on specific components as a cause of flame loss, but it does not say every flame-loss code is caused by scale. The right troubleshooting move is to consider scale as part of the evidence picture, especially when maintenance history is thin or the unit has not been descaled on a reliable cadence.
This is where problem solving and maintenance history meet. If the display pattern points to flame loss and the unit has scale risk, the maintenance record becomes relevant. If the display pattern points elsewhere, the scale fact should not be stretched beyond what it says.
Low Flow Can Create Cold Bursts
Per an industry/contractor source, a tankless burner will not fire below the activation flow, described as about 0.4-0.75 GPM. The source connects that low-flow condition with a cold-water sandwich. Another industry/contractor source defines the cold-water sandwich as hot water, then a cold burst as the burner cycles, then hot water again.
That pattern is different from a total loss of hot water. The heater may be able to produce hot water, but the burner cycling and minimum activation behavior can create an uncomfortable temperature interruption. That is why the symptom description matters. Saying the water goes hot, cold, then hot again is more useful than saying the unit randomly fails.
A minimum-flow explanation fits this specific pattern. It does not support inventing a universal adjustment or a guaranteed repair. The next step is to identify whether the symptom matches the hot-cold-hot sequence and whether the demand may be below the activation flow cited by the industry/contractor source.
This problem is also a reminder that tankless symptoms can be demand-related. The unit may be responding to flow conditions rather than showing a complete equipment failure. Describing the water pattern clearly helps keep that possibility visible.
Frequently asked questions
What does code 11 usually point to on a tankless water heater?
Per an industry/contractor source, Rinnai code 11, Rheem 11, and Navien E003 are described as ignition failure patterns.
What does code 12 usually point to?
Per an industry/contractor source, Rinnai code 12, Rheem 12, and Navien E012 are described as flame failure or flame loss patterns.
Can scale cause flame loss?
Per an industry/contractor source, scale on the heat exchanger, flow sensor, or temperature sensor can cause flame loss.
What is a cold-water sandwich?
Per an industry/contractor source, it is a hot-water pattern where water is hot, then a cold burst appears as the burner cycles, then hot water returns.
Can low flow keep a tankless burner from firing?
Per an industry/contractor source, flow below about 0.4-0.75 GPM can be below activation flow, so the burner will not fire.