So, you’re standing there—maybe in a towel, maybe elbow-deep in dirty dishes—just trying to get some hot water when BAM, your trusty Navien tankless water heater flashes an error code on the display like it’s trying to communicate in Morse code.
And you’re thinking:
“What the heck is error code E110? Should I be worried? Is my hot water gone forever?”
Relax. You’re not alone—and most of the time, these codes are less about doom and more about diagnostics. Navien water heaters are actually pretty smart systems. They’re designed to throw up error codes so you can catch issues early, and in many cases, you can either fix them yourself or know exactly what to tell your plumber.
Let’s break it all down—code by code, symptom by symptom, with plenty of tips and what-to-dos along the way.
First, What’s the Deal with Navien Error Codes?
Navien tankless units are compact, high-efficiency hot water machines. But because they’re running advanced electronics and safety systems, they’ll flash error codes on the front display whenever something goes off course—whether it’s a dirty air filter, a frozen pipe, or a sensor on the fritz. Each code has a number (like E003, E110, or E438) that correlates to a specific problem. Some are simple fixes. Others may require a technician.
Common Navien Tankless Water Heater Error Codes & What They Mean
Here’s a cheat sheet of the most common error codes you’ll see and what’s likely going on behind the scenes.
E003 – Ignition Failure
This is one of the most common ones. Basically, the unit tried to fire up the burner and… nothing happened.
Possible causes:
- Gas supply issue (low pressure or valve turned off)
- Dirty flame rod or igniter
- Air in the gas line (especially after gas was shut off)
- Clogged intake/exhaust
How to Fix:
- Check that the gas valve is fully open.
- Try resetting the unit (power off/on).
- If it persists, call a pro to check the flame sensor and burner assembly.
E012 – Flame Loss
This means the unit ignited the flame, but couldn’t keep it going.
Possible causes:
- Gas supply interruption
- Flame sensor is dirty
- Strong backdraft (wind blowing into exhaust)
How to Fix:
- Make sure there’s no obstruction in the venting.
- Check that the flame rod is clean (or have it serviced).
- Double-check gas supply.
E110 – Abnormal Air Pressure
Translation: Your unit thinks something’s wrong with the airflow.
Possible causes:
- Blocked intake or exhaust pipe (bird’s nest, snow, debris)
- Vent pipes too long or not to spec
- Blower fan issue
How to Fix:
- Go outside and check both intake and exhaust pipes—make sure nothing’s blocking them.
- If it’s super windy or icy out, that can cause temporary airflow problems.
- If it keeps happening, a tech should check the fan and pressure sensor.
E351 – Incoming Water Temperature Too Low
Your Navien is detecting really cold incoming water—often below freezing.
Possible causes:
- Frozen pipes
- Very cold climate, poor insulation on pipes
- Long standby time in extreme cold
How to Fix:
- Check for frozen sections of pipe. Use a hairdryer or space heater to thaw carefully.
- Insulate your water lines and install a recirculation pump if necessary.
- Keep a small drip running on cold nights to prevent freezing.
E438 – Water Leak Detection
Yikes—this one sounds serious because it is. The internal leak detection system thinks there’s water where it shouldn’t be.
Possible causes:
- A legitimate leak inside the unit
- Condensation build-up
- Loose connections
How to Fix:
- Turn off power and water to the unit.
- Check the base of the unit for signs of moisture or pooling.
- If you spot any, call a tech ASAP. This one’s not a DIY situation.
E302 – Hot Water Outlet Thermistor Open or Short
That’s a fancy way of saying your unit’s temperature sensor isn’t reading correctly.
Possible causes:
- Faulty or disconnected thermistor
- Wiring issue
- Sensor got wet or damaged
How to Fix:
- Power cycle the unit.
- If the code returns, the thermistor may need to be replaced.
E407 – Flow Sensor Malfunction
Your unit is having trouble measuring how much water is flowing through it.
Possible causes:
- Mineral buildup on the sensor
- Clogged filter or heat exchanger
- Faulty sensor
How to Fix:
- Clean the cold water inlet filter (located just where water enters the unit).
- If you have hard water, consider a descaling treatment or water softener.
All Other Navien Error Codes: Meaning, Causes, and Solutions
| Error Code | Meaning | Possible Causes | Suggested Actions |
|---|---|---|---|
| E001 | Water boiling in heat exchanger | Clogged inlet filter, scale buildup, faulty thermistor | Clean the inlet filter, flush the heat exchanger with a descaling solution, check the thermistor |
| E002 | Low water pressure | Closed valve, clogged filter, insufficient water supply | Open the valve, clean or replace the filter, ensure adequate water supply |
| E003 | Ignition failure | Gas supply issues, dirty flame sensor, faulty igniter | Check gas supply, clean the flame sensor, inspect the igniter |
| E004 | False flame detection | Grounding issues, dirty flame sensor, faulty igniter | Inspect grounding, clean the flame sensor, check the igniter |
| E005 | Heating supply outlet thermistor open | Loose connections, faulty thermistor | Check connections, replace the thermistor if necessary |
| E006 | Heating supply outlet thermistor short | Short circuit, water ingress | Inspect for shorts, replace the thermistor |
| E007 | Hot water outlet thermistor open | Loose connections, faulty thermistor | Check connections, replace the thermistor |
| E008 | Hot water outlet thermistor short | Short circuit, water ingress | Inspect for shorts, replace the thermistor |
| E009 | Abnormal fan motor activity | Dirty fan, faulty motor, loose connections | Clean the fan, check motor operation, secure connections |
| E010 | Abnormal air pressure | Blocked vent, dirty air filter, faulty pressure switch | Clear vent, clean air filter, inspect pressure switch |
| E011 | High water pressure | Faulty pressure sensor, system overpressure | Check pressure sensor, adjust system pressure |
| E012 | Flame loss | Gas supply issues, dirty flame sensor, faulty igniter | Check gas supply, clean the flame sensor, inspect the igniter |
| E013 | Abnormal pump | Air in the system, faulty pump | Bleed the system, check pump operation |
| E015 | Abnormal control board | Power supply issues, faulty control board | Inspect power supply, replace control board if necessary |
| E016 | Overheating of heat exchanger | Clogged filter, scale buildup, faulty thermistor | Clean the filter, flush the heat exchanger, check the thermistor |
| E018 | Heating return inlet thermistor open | Loose connections, faulty thermistor | Check connections, replace the thermistor |
| E019 | Heating return inlet thermistor short | Short circuit, water ingress | Inspect for shorts, replace the thermistor |
| E021 | Cold water inlet thermistor open | Loose connections, faulty thermistor | Check connections, replace the thermistor |
| E022 | Cold water inlet thermistor short | Short circuit, water ingress | Inspect for shorts, replace the thermistor |
| E027 | Abnormal activity of air pressure sensor | Blocked vent, dirty air filter, faulty sensor | Clear vent, clean air filter, inspect sensor |
| E028 | Water leak | Loose connections, internal leak | Inspect for leaks, tighten connections |
| E030 | Exhaust overheat | Blocked vent, dirty air filter, faulty thermistor | Clear vent, clean air filter, check thermistor |
| E035 | Abnormal activity of gas pressure sensor | Gas supply issues, faulty sensor | Check gas supply, inspect sensor |
| E036 | Communication failure | Loose connections, faulty control board | Check connections, replace control board |
| E039 | Abnormal water flow sensor | Clogged filter, faulty sensor | Clean the filter, inspect sensor |
| E040 | Outdoor thermistor short | Short circuit, water ingress | Inspect for shorts, replace the thermistor |
| E048 | Abnormal LPG gas pressure | Low gas pressure, faulty regulator | Check gas pressure, inspect regulator |
| E046 | Ignition or flame failure | Gas supply issues, dirty flame sensor, faulty igniter | Check gas supply, clean the flame sensor, inspect the igniter |
| E047 | Abnormal exhaust thermistor open or short | Loose connections, faulty thermistor | Check connections, replace the thermistor |
| E060 | Abnormal dual venturi limit switch operation | Faulty limit switch, blocked vent | Inspect limit switch, clear vent |
| E109 | Abnormal fan motor activity | Dirty fan, faulty motor, loose connections | Clean the fan, check motor operation, secure connections |
| E110 | Exhaust blockage | Blocked vent, dirty air filter, faulty fan | Clear vent, clean air filter, inspect fan |
| E127 | Flue temperature sensor malfunction | Faulty sensor, loose connections | Inspect sensor, check connections |
| E205 | Communication failure with remote | Loose connections, faulty remote | Check connections, replace remote |
| E218 | Incomplete burner ignition | Gas supply issues, dirty burner, faulty igniter | Check gas supply, clean burner, inspect igniter |
| E302 | Hot water outlet thermistor open or short | Loose connections, faulty thermistor | Check connections, replace the thermistor |
| E352 | Water pressure too low | Closed valve, clogged filter, insufficient water supply | Open the valve, clean or replace the filter, ensure adequate water supply |
| E353 | Flow sensor malfunction | Clogged filter, faulty sensor | Clean the filter, inspect sensor |
| E407 | Abnormal hot water outlet thermistor open or short | Loose connections, faulty thermistor | Check connections, replace the thermistor |
| E421 | Abnormal cold water inlet thermistor open or short | Loose connections, faulty thermistor | Check connections, replace the thermistor |
| E480 | Gas valve failure | Faulty gas valve, loose connections | Inspect gas valve, check connections |
| E515 | Abnormal PCB communication | Loose connections, faulty PCB | Check connections, replace PCB |
| E517 | Abnormal dip switch setting | Incorrect settings, faulty dip switch | Adjust settings, inspect dip switch |
| E593 | Abnormal panel key | Faulty panel key, loose connections | Inspect panel key, check connections |
| E594 | Abnormal input data | Faulty sensor, loose connections | Inspect sensors, check connections |
| E615 | Blocked vent | Blocked vent, dirty air filter | Clear vent, clean air filter |
| E740 | System shutdown due to overload | High demand, internal failure | Reduce demand, inspect system |
| E782 | Internal short circuit | Faulty wiring, short circuit | Inspect wiring, address short circuit |
| E736 | Cascade communication failure | Loose connections, faulty cascade unit | Check connections, replace cascade unit |
| E760 | Flushing/service alarm | Required maintenance overdue | Perform maintenance |
| E785 | Heat exchanger malfunction | Scale buildup, faulty exchanger | Flush heat exchanger, inspect exchanger |
| E432 | Abnormal cold water inlet 2 thermistor open or short | Loose connections, faulty thermistor | Check connections, replace the thermistor |
| E434 | Abnormal water adjustment valve open or short | Faulty valve, loose connections | Inspect valve, check connections |
| E439 | Abnormal flow sensor | Clogged filter, faulty sensor | Clean the filter, inspect sensor |
| E441 | Abnormal hot water inlet 2 thermistor open or short | Loose connections, faulty thermistor | Check connections, replace the thermistor |
| E445 | Abnormal mixing valve open or short | Faulty valve, loose connections | Inspect valve, check connections |
Bonus: Simple Maintenance to Avoid Error Codes
Want fewer error codes? Preventive care is where it’s at. Navien systems are reliable—but like any appliance, they need love. Here’s what you can do:
✅ Clean the inlet water filter
It catches debris before it enters the heat exchanger. Check it every 6–12 months.
✅ Flush the heat exchanger
If you have hard water, scale builds up over time. Use a vinegar flush kit or descaler every 1–2 years.
✅ Keep intake and exhaust vents clear
Snow, leaves, birds—don’t let anything block those pipes.
✅ Install a surge protector
Electrical surges or brownouts can confuse the control board and throw random codes.
✅ Check for firmware updates
Some Navien models allow software updates through service techs. These updates can fix bugs and improve performance.
Also Read: Navien Error Code E762
When to DIY vs When to Call a Pro
DIY it if:
- It’s a simple fix like clearing vents or checking the water filter.
- The manual says it’s user-serviceable.
- The error goes away after a reset and doesn’t return.
Call a pro if:
- The code returns persistently.
- It involves gas, electronics, or internal parts.
- You see visible leaks or burnt components.
- You have any doubt about what you’re doing.
Seriously, there’s no shame in bringing in a licensed plumber or HVAC tech. Better safe than scorched or soaked.
Don’t Let Error Codes Freak You Out
Getting an error code on your Navien tankless water heater can feel like your home just got a little too smart for its own good—but honestly, these codes are a gift. They’re your water heater’s way of whispering (or yelling), “Hey, something’s up. Please check me out.” Once you know what the codes mean and how to react, you’ll save time, money, and stress. Plus, you’ll earn some serious homeowner cred. So the next time your Navien flashes you a cryptic error code? Take a breath, grab this guide, and show it who’s boss.
