Navien · Tankless Water Heater Critical
E016 — Overheat / High-Limit Trip
The high-limit switch or overheat sensor detected unsafe heat-exchanger temperature. The burner is locked out until reset.
- Keep using it?
- No — service first
- DIY difficulty
- Moderate
- Verified against
- navieninc.com ↗
How to fix it
- Reset the unit; if it trips again under heavy draw, suspect scale or flow.
- Descale the heat exchanger with white vinegar or a tankless cleaner via the service ports (3-4 hour circulation).
- Clean the cold-side inlet filter screen (it traps debris that chokes flow).
- Inspect the venting for blockage, nests, or sagging that traps condensate.
- Test the high-limit fuse/switch for continuity; replace if open at room temp.
- Check outlet thermistor resistance against spec; replace if drifted.
- If symptoms persist after descale and parts check, replace the flow sensor.
Why it happens
Typical signs: Hot water then sudden shutdown · E016 on display · Heat exchanger hot to the touch · Repeated lockouts on long draws
- Scale buildup in heat exchanger Calcium scale insulates the exchanger and drives wall temps up — most common cause in hard-water areas.
- Restricted water flow Clogged inlet screen, partially closed isolation valve, or fouled aerators reduce flow below safe threshold.
- Failed flow sensor / turbine Under-reporting flow lets the burner over-fire for the actual GPM.
- Blocked flue or intake Reduced combustion airflow causes incomplete heat transfer and exchanger overheating.
- Failed high-limit switch Open or drifting switch trips prematurely.
- Failed outlet thermistor Mis-reading temp causes runaway firing.
Parts you may need
If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.
More Navien tankless water heater codes
Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: navieninc.com.