Navien E421 Error Code: Cold water inlet 1 thermistor open or short circuit
Updated October 2026 · Sourced from Navien documentation
🛑 Turn it off and call a licensed pro
The cold water inlet 1 temperature sensor circuit is open or shorted. The manual says to contact customer support.
Is it safe to keep running? No. Turn it off and get it checked. Call a licensed technician or Navien customer support (1-800-519-8794, per the manual). Do not open the unit.
E421 at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| E421 Error code shown on the front panel display | Cold water inlet 1 thermistor open or short circuit | NPE-180A2, NPE-210A2, NPE-240A2, NPE-180S2, NPE-210S2, NPE-240S2 condensing tankless water heaters |
Check this first
Only steps a homeowner can do safely. Do not open electrical panels or the burner area.
- Turn the unit off with the Power button, wait a minute, then turn it back on to see if the code clears.
🛑 Stop and call a licensed pro if
- Call a licensed technician or Navien customer support (1-800-519-8794, per the manual). Do not open the unit.
- You smell gas. Leave the house, do not touch switches, and call your gas utility from outside.
- Codes for flame failure, overheating, venting or the control board that return after one reset.
- Water leaking from the heater, the relief valve discharging, or scorching around the burner area.
- A carbon monoxide alarm sounds. Get everyone outside and call 911 or the fire department.
Safe first steps for any Navien code
- Write down the exact code shown on the controller or the number of status light flashes.
- Turn the water heater power off and back on (or press the reset button the manual describes) once. If the code comes back, stop resetting.
- Make sure the gas supply valve to the heater is open and other gas appliances in the home are working.
- Make sure the cold water supply valve is fully open and hot water taps are not partly closed.
- Look at the outside vent terminal for snow, ice, leaves or anything blocking it (look only).
Source: Navien NPE Condensing Water Heaters User's Information Manual (NPE-2, rev 201202), 5.2 Understanding Error Codes, p.17. Codes are series-specific, so your unit's own manual is the final word. Reviewed October 2026.
Ryan Giles · Red Seal Certified Plumber · Ask a plumber · Ask an HVAC technician
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