Tankless Water Heater Error Codes Explained: A Brand-by-Brand Guide
A brand-by-brand guide to tankless water heater error codes for Rinnai, Navien, Noritz, and Rheem. Learn what each code means, why it triggers, and when to call a pro.
Tankless Water Heater Error Codes Explained: A Brand-by-Brand Guide
A blinking display and an unfamiliar code are among the most stressful things a water heater can show you, especially when you're expecting hot water. The good news is that tankless water heater error codes are designed to tell you exactly what went wrong. Once you know how to read them, most codes point to a specific system, and many can be resolved without a service call.
This guide covers error codes for the four most common tankless brands in U.S. homes (Rinnai, Navien, Noritz, and Rheem), explains the underlying causes, and walks through what to do first before picking up the phone.
- Tankless error codes fall into four main categories: ignition failures, exhaust/venting issues, water flow problems, and overheating
- Most codes clear after you fix the underlying problem and press the reset button
- Recurring codes after a reset almost always mean the root cause hasn't been addressed
- Codes related to exhaust or combustion should be treated as priority repairs
- Annual descaling and filter cleaning prevent the majority of flow and sensor errors
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How to Read a Tankless Error Code
Unlike traditional tank water heaters that show their age silently, tankless units have a digital control board that monitors every subsystem. When something falls outside acceptable parameters (water flow, exhaust pressure, flame signal, temperature), the unit locks out and displays a code. That code is a starting point, not a final verdict.
Most tankless units store the last several error codes in memory, even after a reset. A technician can pull this history using the unit's diagnostic mode. If you're having recurring issues, writing down the codes you see (and in what order) gives a service technician a significant head start.
Rinnai Error Codes
Rinnai is one of the most widely installed tankless brands in the U.S. Their error codes use a two-digit number format. Here are the most common:
| Error Code | Meaning | Common Cause | DIY Fix? |
|---|---|---|---|
| Error 11 | No ignition | Gas supply shut off, low gas pressure, or igniter failure | Partial |
| Error 12 | Flame failure | Flame lit but extinguished; gas valve or flame sensor issue | No |
| Error 14 | Thermal fuse tripped | Overheating from blocked venting or scale buildup | No |
| Error 16 | Outgoing water overheating | Thermostat set too high; scale in heat exchanger | Partial |
| Error 25 | Condensate neutralizer overflow | Condensate trap full or drain blocked (condensing models) | Yes |
| Error 31 | Cold water inlet sensor failure | Faulty or disconnected temperature sensor | No |
| Error 32 | Outgoing water sensor failure | Faulty or disconnected temperature sensor | No |
| Error 33 | Heat exchanger sensor failure | Sensor malfunction or heat exchanger scaling | No |
| Error 41 | Abnormal combustion air fan | Fan blocked, failed, or not at target speed | No |
| Error 52 | Modulating gas valve issue | Gas valve not responding correctly to control board | No |
| Error 61 | Exhaust fan motor failure | Fan motor failed or venting obstruction detected | No |
| Error 65 | Water flow control failure | Flow sensor or water valve malfunction | No |
| Error 71 | Solenoid valve failure | Bypass or gas solenoid valve not functioning | No |
Navien Error Codes
Navien condensing tankless units (NCB and NPE series) use a similar numeric code system. Navien codes tend to be more detailed, with E codes for errors and W codes for warnings:
| Error Code | Meaning | Common Cause | DIY Fix? |
|---|---|---|---|
| E001 | Ignition failure | Gas supply off or insufficient; ignition electrodes dirty or worn | Partial |
| E002 | Abnormal combustion | Incorrect gas type, blocked flue, or inadequate combustion air | No |
| E003 | Ignition overload | Too many failed ignition attempts; lockout protection engaged | Partial |
| E004 | False flame detection | Flame sensor reading flame when unit is off; sensor or wiring fault | No |
| E012 | Flame loss during operation | Gas pressure drop or faulty gas valve | No |
| E030 | Exhaust high limit | Flue blockage, recirculation of exhaust gases, or dirty heat exchanger | No |
| E033 | Inlet water temperature sensor | Sensor open or short circuit; sensor replacement needed | No |
| E046 | Abnormal air pressure | Blocked exhaust or air intake; condensate drain blocked | Partial |
| E109 | Cold water inlet flow sensor | Flow sensor dirty or failing; inlet filter clogged | Partial |
| W003 | Maintenance reminder | Annual descaling or filter service is due | Yes |
Noritz Error Codes
Noritz uses a two-digit error code system with a blinking light pattern on older models or a digital display on newer ones. Many Noritz codes overlap with Rinnai codes in number since both brands trace roots to similar engineering platforms:
| Error Code | Meaning | Common Cause | DIY Fix? |
|---|---|---|---|
| Code 10 | Exhaust abnormality | Blocked or inadequate venting; bird nest or debris in flue | Partial |
| Code 11 | Ignition failure | No gas supply, low pressure, or ignition electrode issue | Partial |
| Code 12 | Flame loss | Gas supply interrupted during operation | No |
| Code 16 | Hot water outlet overheating | Scale buildup in heat exchanger; very low flow rate | Partial |
| Code 20 | High-limit thermal fuse | Overheating from scale or blocked venting; fuse may need replacement | No |
| Code 29 | Condensate drain abnormality | Condensate drain clogged or frozen (condensing models) | Yes |
| Code 51 | Gas solenoid valve issue | Gas valve not opening or closing correctly | No |
| Code 52 | Modulating gas valve | Gas valve modulation sensor or circuit failure | No |
| Code 70 | Control board failure | Internal PCB fault; board replacement likely needed | No |
Rheem Tankless Error Codes
Rheem tankless units (including their EcoSmart and Richmond-branded models) use a letter-number format for error codes. Rheem's Econet-connected models also push error notifications to the Rheem app:
| Error Code | Meaning | Common Cause | DIY Fix? |
|---|---|---|---|
| C1 | No ignition | Gas not reaching burner; check supply shutoff and pressure | Partial |
| C2 | Abnormal combustion | Incorrect gas type or pressure; blocked venting | No |
| F1 | Cold water inlet sensor | Inlet temperature sensor open or shorted | No |
| F2 | Hot water outlet sensor | Outlet temperature sensor open or shorted | No |
| F5 | Heat exchanger overheating | Scale buildup in heat exchanger; annual flush needed | Partial |
| P1 | Cold water flow rate low | Inlet filter clogged; flow below minimum activation threshold | Yes |
| P5 | Recirculation pump fault | Recirculation pump failure or wiring issue | No |
| IL | Scale buildup warning | Descaling service overdue; may cause F5 if ignored | Yes |
Error Codes by Problem Category
Regardless of brand, most tankless error codes fall into one of five root causes. Understanding which category your code belongs to helps you prioritize the response:
First check that the gas supply valve is fully open. Verify that other gas appliances in the home are working. Low gas pressure from the utility or a regulator issue will cause these codes across all brands. Do not repeatedly attempt ignition.
Usually caused by scale buildup in the heat exchanger insulating the water from heat transfer, or a blocked exhaust causing hot gases to back up into the unit. Annual descaling prevents most overheating codes.
Inspect the intake and exhaust termination points on the outside wall. Snow, debris, bird nests, and even spider webs can partially block the flue enough to trigger a shutdown. This is one of the few categories with a legitimate DIY first step.
Flow-related codes often mean the inlet filter screen is clogged with sediment or the flow rate has dropped below the unit's minimum activation threshold (typically 0.5 GPM). Clean the inlet filter first before assuming a sensor has failed.
Sensor and control board failures are almost always professional repairs. A good technician will test sensor resistance with a multimeter before recommending a board replacement, since sensors are much less expensive than boards.
What to Do When an Error Code Appears
Before calling a technician, run through this sequence for any error code. It resolves a surprising number of service calls for free:
- Note the exact code. Write down the code before touching anything. Some units cycle through multiple codes and the first one displayed is the most important.
- Check the gas supply. Confirm the gas shutoff valve at the unit is fully open. Check whether other gas appliances (range, furnace) are working normally. If they're not, the issue is upstream at the meter or regulator.
- Inspect the air intake and exhaust terminations. Go outside and visually check both the intake and exhaust pipe ends. Look for debris, ice, or nesting material blocking either opening.
- Check the inlet filter screen. Most tankless units have a small mesh filter at the cold water inlet connection. Turn off the water supply, remove the screen, rinse it clean, and reinstall. A partially blocked screen can trigger flow codes and even ignition codes.
- Reset the unit. Press the reset button (usually on the front panel or remote). If the code returns within one heating cycle, the root cause has not been resolved and resetting again won't help.
- Check for a condensate drain blockage (condensing models only). On Navien, Rinnai RUR, and Noritz NRCP models, a blocked condensate drain triggers a pressure switch fault. Pour a small amount of water into the condensate pan and verify it drains freely.
- Call a licensed technician. If the code returns after a reset and you can't identify an obvious cause, it's time for a professional diagnosis. Provide the technician with the exact code, how often it occurs, and whether it happens at startup or during operation.
If your error code involves ignition, flame loss, abnormal combustion, or exhaust (Rinnai 11/12/61, Navien E001/E002/E030, Noritz 10/11/12, Rheem C1/C2), do not attempt more than two resets. Repeated failed ignition attempts can flood the combustion chamber with unburned gas. Turn the unit off and call a licensed technician or your gas utility.
How Scale Buildup Triggers Error Codes
The single most common underlying cause of recurring tankless error codes is calcium and magnesium scale in the heat exchanger. Unlike tank water heaters where sediment settles at the bottom, tankless units concentrate heat through narrow copper or stainless channels. Scale coats those channels, reduces heat transfer, forces the unit to run hotter to compensate, and eventually triggers overheating shutdowns.
The U.S. Department of Energy recommends descaling tankless units annually in hard water areas. A descaling procedure circulates a diluted food-grade citric acid or white vinegar solution through the heat exchanger for 45 to 60 minutes, dissolving calcium deposits before they harden into scale that triggers error codes. This is a core maintenance step that prevents the majority of F5, Error 16, Error 14, and overheating codes.
If you're not sure how hard your water is, check our guide on sediment buildup signs and water hardness levels. In very hard water areas (above 7 GPG), consider a whole-house water softener to dramatically reduce scale formation and extend the life of your tankless unit.
When to Repair vs. Replace
Tankless water heaters are designed to last 20 or more years with proper maintenance, so most error codes are worth repairing if the unit is under 12 to 15 years old. The exceptions are:
- Control board failure (Noritz Code 70, Navien board fault codes): Board replacements can cost $400 to $800 in parts alone. On a unit older than 12 years, that cost approaches replacement territory.
- Heat exchanger replacement: If the heat exchanger is cracked or leaking due to severe scale damage, replacement costs $600 to $1,200 or more. On older units, a new heat pump or high-efficiency tankless unit may make more financial sense.
- Recurring codes despite repairs: If the same error code returns within 6 months of a repair, the technician either misdiagnosed the root cause or the unit has a deeper systemic issue.
Our guide on repair vs. replace walks through the full decision framework. Use our Replacement Cost Calculator to estimate what a new tankless unit would cost for your home.
Preventing Error Codes: Annual Maintenance Checklist
- Clean the cold water inlet filter screen
- Inspect the exhaust termination outside
- Check the condensate drain for blockage (condensing units)
- Flush/descale the heat exchanger (especially in hard water areas)
- Inspect the venting connections for corrosion or separation
- Test the pressure relief valve
- Check for error code history in the diagnostic menu
- Have a licensed technician inspect gas connections and burner assembly
- Test the igniter and flame sensor
- Verify combustion efficiency with a combustion analyzer
Frequently Asked Questions
Can I reset a tankless water heater error code myself?
Yes, for most codes you can press the reset button on the unit or the remote controller after addressing the underlying issue. The key word is "after." Resetting without fixing the cause is a short-term workaround; the code will return. For combustion and exhaust codes, limit resets to two attempts and call a technician if the code returns.
Why does my tankless unit show an error code only in cold weather?
Cold weather error codes are usually venting or condensate issues. In freezing temperatures, the condensate drain can freeze and block drainage, triggering a pressure switch fault. The exhaust and intake terminations can also accumulate ice. Some units have a built-in freeze protection mode that activates the recirculation pump; if the pump isn't working, a freeze-related code may follow. Insulating the condensate drain line and ensuring the vent terminations are above any expected snow depth prevents most cold-weather codes.
What does it mean when my tankless unit shows LC0, LC1, or LC (scale warning)?
This is a descaling reminder code used by Navien and some other brands. LC0 means the unit is approaching the service interval, and LC1 (or LC) means the descaling service is now due. Ignoring it for too long allows scale to build until it triggers an overheating shutdown (E030 or similar). Schedule a descaling service as soon as you see these codes.
My tankless water heater shows Error 11 or E001 but I have gas. What else could cause it?
Even when gas is present, ignition can fail for several reasons: the gas pressure may be within the acceptable range for the appliance but low enough that the burner can't sustain ignition; the igniter itself may be worn and not producing a strong enough spark; the flame sensor may be coated with oxidation and not confirming ignition to the control board; or the venting may be creating enough back-pressure to blow out the flame before the sensor confirms it. If confirming gas supply doesn't clear the code, the diagnosis requires a technician with a manometer and combustion test tools.
How much does it cost to fix a tankless water heater error code?
Costs vary widely by the code and the cause. A simple filter cleaning or condensate drain clearing might cost $75 to $150 for a service call if you can't do it yourself. Sensor replacements run $150 to $350 including labor. Gas valve or fan motor replacements are $300 to $600. Control board replacements are $400 to $900. If your unit is old enough that these repairs push past 50% of replacement cost, it may be time to weigh a new unit instead. Use our Replacement Cost Calculator to compare.
Does an error code void my warranty?
The code itself doesn't void the warranty, but how you respond to it might. If a combustion or overheating code is repeatedly reset without addressing the cause, and the heat exchanger is eventually damaged, manufacturers can argue that continued operation after a safety shutdown constitutes misuse. Keep a log of codes you see and when, and if you have a technician service the unit, get a written record of what was found and repaired. See our guide on what voids a water heater warranty for more detail.
Bottom line: A tankless error code is your unit talking. It's telling you exactly which system needs attention, and most codes have a logical path to resolution. Start with gas supply and venting checks, clean the inlet filter, and reset once. If the code returns, the diagnosis goes deeper than a DIY fix. Annual descaling, quarterly filter cleaning, and keeping the venting clear will prevent most codes from ever appearing in the first place. Not sure which brand or model you have? Decode your serial number here to identify your unit before troubleshooting.