Navien Error Code E421: Dom.Cold Water Inlet Sensor
Severity
Warning
Type
digital display
Estimated repair cost:$100 — $350(parts + labor)
Warning: The Navien tankless water heater has shut down due to a cold water inlet sensor issue, meaning you won't have hot water until it's fixed.
How to Fix Navien Code E421
Step 1: Check the water supply. Make sure the cold water supply valve to the Navien is fully open.
Step 2: Power cycle the unit. Turn off the power to the Navien at the breaker for 30 seconds, then turn it back on. This might reset a temporary glitch.
Step 3: If the error persists after these steps, contact a qualified HVAC technician to diagnose and repair the issue.
How to Reset Navien Code E421
Turn off the unit using the power button on the front panel.
Turn off the gas supply valve.
Turn off the circuit breaker supplying power to the unit. Wait 30 seconds.
Restore power at the breaker, then open the gas valve.
Turn the unit back on. If the error returns, the underlying issue needs repair.
What does E421 mean?
The E421 error code on your Navien tankless water heater indicates a problem with the Domestic Cold Water Inlet Sensor. This sensor monitors the temperature of the incoming cold water. The Navien control board uses this information to regulate the burner and maintain a consistent hot water temperature. If the sensor fails or provides an incorrect reading, the unit will shut down to prevent potential overheating or other issues. Basically, the unit doesn't trust the temperature reading it's getting, so it's playing it safe by locking out. This protects the unit from damage.