Navien Tankless Water Heater Error Codes — E001, E002, E016, E021, E110, E407 Complete Guide

Common Symptoms

• Error code displayed on Navien front panel (E001, E002, E016, E021, E110, E407, or other)
• No hot water — unit locks out and displays error code
• Intermittent hot water — unit ignites then loses flame, E110 on display
• Unit fires but output temperature is limited — E016 scale overheat protection
• No ignition — burner never lights, E001 or E002 displayed
• Water heater shuts down completely — E407 leak sensor activated

Most Likely Causes

1. 1

   E001 / E002 — Ignition Failure (No Flame Established)

   E001 indicates the first ignition attempt failed. E002 indicates the unit attempted ignition 3 times without establishing a flame and locked out. First check: verify gas supply — are other gas appliances (stove, dryer) in the home working? If not, the gas supply to the house has been interrupted. At the unit: the inlet gas pressure for natural gas should be 3.5 inches water column (WC) minimum at the unit connection point under full flow; propane requires 8 inches WC. Low gas pressure from undersized supply lines or too many appliances on the same meter is a common cause. Test the igniter for continuity — disconnect the igniter lead and measure resistance, should read 30–100Ω depending on model. Clean the flame sensor rod with fine steel wool — carbon deposits on the flame sensor rod cause false 'no flame' detection even when the burner lights.

2. 2

   E016 — Over-Temperature Cutoff (Scale Buildup in Heat Exchanger)

   E016 means the heat exchanger outlet temperature exceeded the safety limit (typically 220°F). In hard water areas, calcium and magnesium deposits build up inside the heat exchanger tubes over 1–3 years, restricting flow and causing local hot spots. Navien recommends annual descale using white vinegar circulation through the heat exchanger isolation valves. The descale procedure requires two isolation ball valves on the hot and cold connections at the unit (if not installed, this is the time to add them), a submersible pump, and 3–4 gallons of undiluted white vinegar circulated for 45 minutes. After descale, flush with clean water for 5 minutes. If E016 returns quickly after descale on a very hard water installation, consider installing an inline scale filter or water softener upstream of the unit.

3. 3

   E021 — Air Pressure Fault (Condensate Drain Line Blocked)

   E021 is a combustion air/exhaust pressure switch fault — the switch detects that flue gas pressure is outside the expected range. The most common cause is a blocked condensate drain line, not a failed pressure switch. Navien condensing tankless heaters produce condensate (acidic water) that drains through a small drain tube to a floor drain. If this tube is kinked, frozen, or blocked with scale, back-pressure builds and trips the E021 pressure switch. Check the condensate drain line: it must slope continuously downward from the unit to the floor drain with no kinks or high points. Also inspect the inlet air and exhaust vent pipes for blockages — bird nests, debris, or ice accumulation at the termination cap (especially in winter) can also trip E021.

4. 4

   E110 — Flame Loss During Operation

   E110 means the unit successfully ignited but lost the flame during the heating cycle. Unlike E001/E002 which are about establishing ignition, E110 occurs after the burner is running. Common causes: gas pressure fluctuation during high-demand periods (pressure drops when multiple appliances run simultaneously), flame sensor rod coated in deposits (clean with fine steel wool), gas valve not opening fully (E060 may accompany E110), or combustion air issues from a partially blocked exhaust. Check gas supply pressure at the unit under load by measuring the manifold pressure — natural gas should maintain 3.5 inches WC under full firing.

5. 5

   E407 — Leak Sensor Activated

   E407 means the unit's internal leak sensor has detected water inside the unit. This is not a sensor fault — it is an actual water leak inside the Navien cabinet. E407 requires immediate response: shut off the cold water supply inlet valve at the unit and close the gas supply shutoff valve. Do not run the unit with E407 active. The leak is most commonly from the pressure relief valve (PRV) discharging (test by reducing supply pressure), a failed internal connection, or a cracked heat exchanger from freeze damage (if the unit is in an unheated space). After shutting off water and gas, open the front cabinet panel to locate the source of the leak before calling for service.

6. 6

   E011 / E010 — Heat Exchanger High Temperature / Exhaust Overheating

   E011 means the heat exchanger temperature sensor detected dangerously high temperature — typically from scale buildup (same root cause as E016) or insufficient water flow through the unit. E010 means the exhaust temperature exceeded limits — usually caused by a blocked exhaust vent. For E011: check the heat exchanger inlet flow — minimum flow rate to fire the unit is typically 0.5–0.75 GPM (model-specific, check the spec sheet). Low-flow fixtures or a partially closed isolation valve can prevent adequate flow. For E010: inspect the entire exhaust vent run for blockages, verify the termination cap is clear, and confirm the vent pipe is the correct diameter and length per the Navien installation manual.

7. 7

   E012 / E030–E033 — Flow or Temperature Sensor Faults

   E012 indicates a flow sensor fault — the flow sensor (turbine-type inside the cold water inlet) is not detecting water flow or is reporting abnormal readings. This can be caused by debris in the flow sensor, a seized turbine wheel, or a failed sensor. E030 covers exhaust temperature sensor faults; E031 covers cold water inlet sensor; E032 covers hot water outlet sensor; E033 covers the heat exchanger sensor. Sensor faults produce resistance values outside the expected range — each sensor is an NTC thermistor. Contact Navien support or check the service manual for resistance specs for each sensor at known temperatures.

Quick DIY Checks

1. 1E001/E002 — verify gas supply before anything else: check whether other gas appliances in the home (gas range, gas dryer, gas furnace) are working normally. If no gas appliances have gas, the issue is with the main gas supply — call the gas utility. If other appliances work but the Navien doesn't fire, check the gas shutoff valve at the unit — it should be fully open (handle parallel to the pipe). For natural gas, the inlet pressure at the unit should read 3.5 inches water column (WC) under firing load; propane requires 8 inches WC. Measure with a manometer at the inlet test port if available. Undersized supply lines drop below minimum pressure when the unit fires at full BTU.
2. 2E001/E002 — clean the flame sensor rod: the flame sensor is a metal rod that sits in the burner flame path and uses ionization current to confirm the flame is present. Carbon and oxidation buildup on the rod surface insulates it and prevents the signal. Shut off the gas and water supply, unplug the unit, and open the front access panel. Locate the flame sensor rod (single rod electrode near the burner assembly). Remove the single mounting screw, pull the rod out, and clean the rod surface with fine steel wool or 400-grit emery cloth until the metal is bright. Reinstall and test ignition. Also check the igniter electrode — it should have a 3–4mm gap to the burner and no visible cracks in the ceramic insulator.
3. 3E016 — perform the Navien heat exchanger descale procedure: the unit requires two isolation service valves on the hot and cold connections (1/2-inch ball valves with hose bib caps, typically installed by the original installer). Shut off the water supply. Close both isolation valves. Connect a small submersible pump to the cold side hose bib port and a return hose from the hot side port back to a 5-gallon bucket containing 3–4 gallons of undiluted white vinegar. Open both isolation valves partway and run the submersible pump for 45 minutes — the vinegar circulates through the heat exchanger and dissolves calcium scale. After 45 minutes, close isolation valves, disconnect the pump setup, flush the heat exchanger with 5 minutes of clean water flow, then restore normal operation. If E016 returns within 3 months, install an inline scale filter.

1. 4E021 — inspect and clear the condensate drain line: locate the condensate drain outlet on the Navien unit — it is a small-diameter (typically 1/2-inch) flexible plastic tube that routes to a floor drain. Trace the entire drain line from the unit to the drain: verify it slopes continuously downward with no uphill sections, kinks, or loops that trap water. Disconnect the drain tube from the unit and blow through it to confirm it is not blocked. Also check the termination end at the floor drain — scale deposits at the tube end can reduce flow. If the condensate drain slopes properly and is clear, inspect the inlet air and exhaust vent termination caps outside — debris, insect nests, or ice accumulation can block combustion airflow and trip the pressure switch.
2. 5E110 — check gas pressure under load and clean flame sensor: E110 (flame loss during operation) most often points to gas pressure dropping below minimum under firing load. Run the unit at maximum flow rate with another gas appliance also running (simulating peak demand) and measure gas pressure at the manifold test port — should maintain 3.5 inches WC (natural gas) throughout the firing cycle. A dip below this causes flame loss mid-cycle. Also clean the flame sensor rod as described in Step 2 — a partially degraded flame sensor may hold the flame signal at ignition but lose it under vibration or thermal expansion once the burner reaches operating temperature.
3. 6E407 — shut down, locate, and identify the leak source: immediately close the cold water supply inlet valve at the bottom of the Navien unit. Close the gas supply valve (quarter-turn ball valve on the gas supply line at the unit). Unplug the unit. Open the front access panel. Look for visible water — wet components, mineral deposits from dried water, or active dripping. Common sources: pressure relief valve (PRV) — the PRV discharges through a downward pipe; if supply pressure exceeds 150 PSI or the PRV seat is degraded, it will discharge continuously. Heat exchanger connections — internal O-rings and brazed joints can fail. Freeze damage — if the unit is in an unheated space (garage), the heat exchanger may have cracked in a freeze event. Document the leak source location and call for service before attempting to operate the unit again.
4. 7E048 — cascade communication fault (multi-unit systems): Navien supports cascade systems linking up to 16 tankless units for commercial or high-demand residential applications. E048 indicates the cascade communication link has failed between units — either a wiring connection fault in the cascade data cable, a failed cascade controller board, or a unit that has locked out on another fault and stopped participating in the cascade sequence. Check the cascade communication cable connections at both units showing E048. Verify each unit in the cascade can run individually (disconnect from cascade and test solo). If a unit fires normally in standalone but faults in cascade, the cascade controller or wiring between units is the fault, not the individual unit. Navien cascade documentation is in the NCB/NPE installation manual.

Repair vs Replace

Recommended Tools & Parts

• Navien Igniter Assembly

  Replacement igniter and flame sensor rod assembly for Navien NR and NPE series. Fixes E001/E002 ignition failure when the igniter element has cracked or the electrode gap is incorrect. Verify model-specific part number — NPE and NR series use different igniters.

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• Submersible Pump + Descale Hose Kit

  Small submersible pump with hose connections for circulating white vinegar through the Navien heat exchanger isolation valve ports. Required for the E016 descale procedure. Reusable for annual maintenance.

  $25–$50

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• Navien Gas Valve

  Replacement combination gas valve for Navien NPE/NR series. Controls gas flow to the burner assembly. Fixes E060 gas valve fault. Verify model-specific part number — gas valve varies between natural gas and propane configurations.

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• Condensate Neutralizer

  Inline condensate neutralizer (limestone chip type) for Navien condensing tankless water heater drain line. Raises condensate pH to safe levels before it reaches the floor drain. Required in many jurisdictions. Install on the condensate drain line from the unit.

  $15–$40

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• Navien Flow Sensor

  Replacement turbine-type flow sensor for Navien NPE/NR series. Fixes E012 flow sensor fault. Located at the cold water inlet inside the unit. Verify part number against model — NR and NPE series use different flow sensors.

  $30–$70

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Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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