What does the Navien E421 fault code mean?

Your boiler is struggling to read its internal water temperature correctly, so it has shut down to prevent overheating or system damage.

How much does it cost to fix Navien error code E421?

Typical repair cost for the Navien E421 fault is £150-250 including labour.

Thermistor failure or inaccurate water temperatures from inside the boiler.

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Lockout fault with a reset-needed pattern.

Safety-critical — turn off, read this.

7 signs repair isn't worth it anymore.