Tankless Water Heater Troubleshooting Guide
Fast answers for Navien, Rinnai, Rheem & Noritz error codes — with clear DIY vs. professional guidance.
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Safety First — Read Before You Begin
Working on tankless water heaters involves high-voltage electricity, natural gas or propane lines, and scalding hot water. Improper repairs can cause serious injury, property damage, or death. Always shut off gas and electricity to the unit before inspection. If you smell gas, leave the area immediately and call your gas utility provider. This guide is for informational purposes only. When in doubt, contact a licensed professional.
Understanding Error Codes
Why Is My Tankless Water Heater Showing an Error Code?
Modern tankless water heaters are sophisticated appliances packed with sensors and diagnostic systems. When something goes wrong — whether it's a gas pressure issue, a faulty flow sensor, an ignition failure, or a venting problem — your unit will display an error code on its LED panel.
These codes are your first clue in the troubleshooting process. Understanding what they mean can save you time and money, and help you determine whether it's a quick DIY fix or a job for a licensed technician.
Find Your Solution
Error Codes by Brand
Select your brand below to find your specific error code, its meaning, and recommended action.
Navien
Rinnai
Rheem
Noritz
| ERROR CODE | MEANING | LIKELY CAUSE | RECOMMENDED ACTION |
|---|---|---|---|
| E001 | Abnormal Ignition | No gas supply, clogged burner, or faulty igniter | Check gas valve is open; call pro if igniter is faulty DIY PRO |
| E003 | Ignition Failure | Gas line issue or ignition electrode dirty/cracked | Clean electrode; call pro for gas issues DIY PRO |
| E004 | False Flame | Flame rod malfunction or wiring issue | Professional inspection recommended PRO |
| E010 | Abnormal Air Pressure | Blocked vent or faulty pressure switch | Check vent for obstructions; replace switch DIY PRO |
| E012 | Flame Loss | Gas supply interrupted during operation | Check gas supply; call pro if persistent DIY PRO |
| E016 | Over-temperature | Heat exchanger overheated | Check flow rate; descale unit; call pro PRO |
| E030 | Exhaust High-Temp | Vent blockage or recirculation | Inspect and clear vent pipe DIY |
| E032 | Cold Water Sandwich | Normal operation variation | No action needed DIY |
| E036 | Abnormal Inlet Thermistor | Faulty inlet temperature sensor | Professional sensor replacement PRO |
| E047 | PC Board Failure | Control board malfunction | Professional board replacement PRO |
| E060 | Abnormal Cascade | Multi-unit communication failure | Check wiring; call pro PRO |
| E109 | Abnormal Gas Pressure | Gas pressure too low or too high | Contact gas provider or pro PRO |
| E110 | Over-temperature Exhaust | Clogged vent or high exhaust temps | Clear venting; call pro DIY PRO |
| ERROR CODE | MEANING | LIKELY CAUSE | RECOMMENDED ACTION |
|---|---|---|---|
| Code 02 | Bypass Servo Driveability | Bypass servo malfunction | Professional repair required PRO |
| Code 03 | Bath Fill Malfunction | Bath fill function error | Check bath fill settings; call pro DIY PRO |
| Code 05 | Solenoid Valve Malfunction | Gas valve failure | Professional replacement required PRO |
| Code 10 | Air Supply/Exhaust Blockage | Blocked vent pipes | Inspect and clear all venting DIY |
| Code 11 | No Ignition | Gas off, igniter failure | Check gas supply; call pro for igniter DIY PRO |
| Code 12 | Flame Failure | Flame lost during operation | Check gas pressure; call pro PRO |
| Code 14 | Thermal Fuse Failure | Overheated unit | Unit requires professional reset/repair PRO |
| Code 16 | Overheat | Heat exchanger too hot | Check water flow; descale; call pro PRO |
| Code 25 | Condensate Neutralizer Maintenance | Condensate needs service | Replace condensate neutralizer DIY |
| Code 31 | Inlet Thermistor Failure | Faulty sensor | Professional sensor replacement PRO |
| Code 32 | Outlet Thermistor Failure | Faulty outlet senso | Professional sensor replacement PRO |
| Code 33 | Heat Exchanger Outlet Thermistor | Faulty sensor | Professional sensor replacement PRO |
| Code 34 | Combustion Air Thermistor | Air sensor failure | Professional repair PRO |
| Code 52 | Modulating Gas Valve Malfunction | Gas valve issue | Professional valve service PRO |
| Code 61 | Combustion Fan Failure | Fan motor issue | Professional motor replacement PRO |
| Code 65 | Water Flow Control Valve | Flow valve failure | Professional replacement PRO |
| Code 70 | PC Board Failure | Control board issue | Professional board replacement PRO |
| Code 71 | Gas Valve Electrical Failure | Wiring or valve issue | Professional inspection PRO |
| Code 72 | Flame Detection Malfunction | Flame sensor issue | Professional repair PRO |
| Code 73 | PC Board/Bath Setup Conflict | Settings mismatch | Reset settings; call pro DIY PRO |
| Code 79 | Water Leak Detected | Internal leak | Shut off water; call pro immediately PRO |
| ERROR CODE | MEANING | LIKELY CAUSE | RECOMMENDED ACTION |
|---|---|---|---|
| C1 00 | No Ignition | Ignition failure | Check gas; call pro for igniter DIY PRO |
| C2 11 | Inlet Thermistor Failure | Faulty temperature sensor | Professional replacement PRO |
| C2 12 | Outlet Thermistor Failure | Faulty outlet sensor | Professional replacement PRO |
| C2 13 | Exhaust Thermistor Failure | Faulty exhaust sensor | Professional replacement PRO |
| C2 21 | High Limit Switch | Overheating detected | Check flow; descale; call pro PRO |
| C4 11 | Inlet Flow Sensor Failure | Flow sensor malfunction | Professional replacement PRO |
| C4 21 | Outlet Flow Sensor Failure | Flow sensor malfunction | Professional replacement PRO |
| C6 11 | Combustion Fan Failure | Fan motor issue | Professional replacement PRO |
| C6 13 | Bypass Flow Sensor Failure | Sensor issue | Professional repair PRO |
| C7 11 | Igniter Failure | Igniter malfunction | Professional replacement PRO |
| C7 14 | Flame Rod Failure | Flame sensor issue | Professional replacement PRO |
| C7 15 | Gas Valve Failure | Valve malfunction | Professional replacement PRO |
| C8 11 | Venting Blockage | Vent blocked or disconnected | Inspect and clear venting DIY |
| FF | Filter Alert | Air filter needs cleaning | Clean or replace filter DIY |
| ERROR CODE | MEANING | LIKELY CAUSE | RECOMMENDED ACTION |
|---|---|---|---|
| Error 10 | Venting/Combustion Air Problem | Blocked or improper vent | Inspect vent; call pro DIY PRO |
| Error 11 | Ignition Failure | Gas off or igniter fault | Check gas; call pro DIY PRO |
| Error 12 | Flame Failure | Flame lost mid-cycle | Check gas; call pro PRO |
| Error 14 | Thermal Fuse Blown | Overheating | Professional reset required PRO |
| Error 16 | Hot Water Overheating | Heat exchanger too hot | Check flow; descale; call pro PRO |
| Error 20 | Water Leak | Internal leak detected | Shut off water; call pro immediately PRO |
| Error 29 | Condensate Sensor | Condensate issue | Check drain; call pro PRO |
| Error 31 | Inlet Sensor Failure | Temperature sensor fault | Professional replacement PRO |
| Error 32 | Outlet Sensor Failure | Outlet sensor fault | Professional replacement PRO |
| Error 33 | Exhaust Sensor Failure | Exhaust sensor fault | Professional replacement PRO |
| Error 34 | Combustion Sensor Failure | Combustion air sensor | Professional replacement PRO |
| Error 52 | Modulation Valve | Gas modulation issue | Professional repair PRO |
| Error 58 | Water Quality Alert | Scale buildup | Descale unit; call pro DIY PRO |
| Error 61 | Fan Motor Failure | Combustion fan fault | Professional replacement PRO |
| Error 63 | Gas Valve Malfunction | Gas valve failure | Professional replacement PRO |
| Error 65 | Flow Sensor Failure | Flow sensor fault | Professional replacement PRO |
| Error 70 | Control Board Failure | PC board malfunction | Professional replacement PRO |
| Error 71 | Solenoid Valve Failure | Gas solenoid issue | Professional repair PRO |
| Error 90 | Combustion Abnormality | Combustion issue | Call pro immediately PRO |
| Error 99 | Fan Malfunction | Fan motor failure | Professional replacement PRO |
Safety & Best Practices
DIY vs. Professional Repair: Know the Difference
Not every error code means you need to call a plumber. Some issues are simple enough to resolve yourself with basic household tools. Others involve gas lines, electrical components, or heat exchangers that require a licensed professional. Here's how to tell the difference.
Safe DIY Fixes
- Checking and opening gas shut-off valves
- Inspecting and clearing venting pipes of obstructions (bird nests, debris)
- Cleaning or replacing the inlet water filter
- Resetting the unit by powering off and back on
- Descaling the heat exchanger with a vinegar flush kit (if experienced)
- Checking water pressure at main supply
- Replacing batteries in the remote controller
- Cleaning air intake filters (Rheem FF code)
Always Call a Professional For
- Any gas line work, gas valve replacement, or gas pressure adjustment
- Igniter or flame rod replacement
- Heat exchanger repair or replacement
- Control board (PC board) failure
- Electrical wiring, thermostat, or sensor replacement
- Error codes involving water leaks (Rinnai Code 79, Noritz Error 20)
- Thermal fuse reset or replacement
- Combustion fan motor replacement
- Any repair when the unit is still under warranty (DIY may void warranty)
Schedule With Confidence
When You Need a Pro, We're Ready
At Tankless Heater Pros, our certified technicians specialize exclusively in tankless water heater installation, repair, and maintenance. We service all major brands — Navien, Rinnai, Rheem, Noritz, Bosch, Takagi, and more — across the region. When your unit throws an error code you can't resolve, or if you simply want peace of mind, we offer same-day diagnostic calls, transparent flat-rate pricing, and a satisfaction guarantee on every job.
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Helpful Answers
Frequently Asked Questions
Quick answers to the most common tankless water heater questions.
How do I reset my tankless water heater?
Turn the unit off at the power switch or unplug it. Wait 30 seconds, then turn it back on. If the error persists, do not continue resetting — investigate the root cause or call a pro.
Why does my tankless heater go cold mid-shower?
This is often caused by a "cold water sandwich" effect in recirculating systems, low gas pressure, a dirty inlet filter, or a failing flow sensor. Check filters first; call a pro if the issue continues.
How often should a tankless water heater be serviced?
Once per year is the standard recommendation. Annual descaling and inspection extends unit life significantly.
Can I flush my tankless water heater myself?
Yes, if you're comfortable with basic plumbing and purchase the correct descaling kit. Always follow your manufacturer's instructions. If your unit has gas connections nearby, consider hiring a pro.
Will my error code go away on its own?
Some codes (like low flow rate errors) may clear once the root cause resolves. Most sensor, gas, or ignition codes will persist and require investigation.
Does an error code void my warranty?
The error itself does not, but performing unauthorized DIY repairs on certain components might. Always check your warranty terms before attempting repairs beyond basic maintenance.
How long do tankless water heaters last?
With proper annual maintenance, most units last 15–20 years — roughly twice the lifespan of a traditional tank heater.
What brands do you service?
We service Navien, Rinnai, Rheem, Noritz, Bosch, Takagi, Eccotemp, Stiebel Eltron, and most other tankless brands.
Tankless Knowledge Center
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