E421 🟡 Repair Required
Navien Error Code E421: Dom.Cold Water Inlet Sensor
⚠️ 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.
What Does Navien Error Code 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.
Common Causes of Navien Code E421
- Faulty Domestic Cold Water Inlet Sensor
- Wiring issue between the sensor and the main control board (loose connection, damaged wire)
- Corrosion on the sensor or its connections
- Sediment or scale buildup on the sensor, affecting its accuracy
- Main control board failure (less common, but possible)
Applicable Navien Models
Source Documentation
This error code is documented in 2 official Navien technical manuals.
Navien NCB-H Installation & Operation ManualPage 129
ReasonDom.Cold Water Inlet Sensor
ActionContact technical support at 1-800-519-8794.
Navien NFC-H Installation & Operation ManualPage 127
ReasonCold water inlet: thermistor open or short circuit
ActionAuto RESET
Need a Pro to Fix Navien Code E421?
A licensed HVAC technician can diagnose and fix this issue quickly.
Find a Licensed Tech Near You →Replacement Parts for Navien Tankless Water Heater Code E421
Cold Water Inlet Sensor OEM: 30010684A
Main Control Board OEM: 30014031A