What does error code E421 mean on a Navien tankless water heater?

Cold water inlet thermistor. A cold water inlet thermistor is open or shorted.

Can I fix Navien code E421 myself?

Start with these safe checks: Note the code. Navien says contact technical support.

When should I call a technician for code E421?

Navien says contact technical support. Smell gas? Leave right away, don't touch light switches or phones inside, and call your gas utility or 911 from outside. Never open a gas valve, burner, gas line, vent or flue connection, or bypass a safety switch. Repeated lockouts, scorching, soot, or a CO alarm mean shut it off and call a licensed gas technician.

Navien tankless water heater error code

Navien tankless water heater code E421 / E432: Cold water inlet thermistor

Navien NPE-2 tankless water heaters

What E421 means

A cold water inlet thermistor is open or shorted.

Also shown as: E432, depending on the model.

Likely causes

Sensor or wiring fault

Safe first checks

Note the code.

When to stop

Navien says contact technical support.

Checked against: Navien NPE-2 user's information manual: understanding error codes. Codes can differ by model and year, so confirm with your owner's manual.