HeatingCodes

Navien NPN tankless water heater error codes

By Jason Dearmin · Last reviewed November 14, 2026

Navien shows an E-prefixed error code on the front panel. The manual's chart gives a reason and a self-diagnostic action for each one, and for many codes the action is simply to contact technical support.

What this table coversCodes from section 5.2 of Navien's NPN non-condensing water heater User's Information Manual. Condensing Navien models use different manuals.

How to read the display

The code appears on the front panel. Some codes (like E003) have a few safe owner checks. Others say only to contact technical support, which you should do rather than opening the unit.

Navien code table

CodeMeaningWhat the document says
E003 Ignition failure Ensure the main gas supply valve is open. Ensure the gas manifold and the fuel DIP switch setting match the supplied gas type. Full explanation
E004 False flame detection Ensure the electrical cord is properly grounded.
E012 Flame loss Clean the intake air filter. Ensure the electrical cord is properly grounded. Ensure the gas manifold and fuel DIP switch setting match the supplied gas type.
E016 Overheating of heat exchanger Turn the system off for at least 30 minutes, then restart it. Clean the cold water inlet filter. Flush the heat exchanger.
E046 Abnormal operation: limit control Contact Navien technical support.
E056 Abnormal operation: modulating gas valve Contact Navien technical support.
E109 Abnormal operation: fan motor Contact Navien technical support.
E110 Abnormal air pressure Ensure the vent pipe is free of obstructions. Full explanation
E127 Abnormal operation: air volume open / short / default value / no answer Contact Navien technical support.
E157 Abnormal operation: air volume control Contact Navien technical support.
E170 Antifreeze heater open / short Contact Navien technical support.
E407 Hot water outlet: thermistor open or short circuit Contact Navien technical support.
E421 Cold water inlet: thermistor open or short circuit Contact Navien technical support.
E434 Abnormal operation: water adjustment valve Contact Navien technical support.
E438 Abnormal operation: circulation pump External: ensure all water supply and return lines are open.
E439 Abnormal operation: flow sensor Contact Navien technical support.
E441 Hot water inlet: thermistor open or short circuit Contact Navien technical support.
E445 Abnormal operation: bypass mixing valve Contact Navien technical support.
E515 Abnormal operation: PCB Ensure the electrical cord is properly grounded.

Source: Navien NPN User's Information Manual, chart in section 5.2.

Detailed explanations

SafetyThese tables describe what the manufacturer documents say. Gas, combustion, refrigerant and line-voltage repairs are for licensed professionals. If you smell gas, leave the building and call your gas utility or 911. Safety disclaimer.

Sources