Navien Error Code E421: Dom.Cold Water Inlet Sensor
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.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
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
- 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)
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| Unit displays E421 error | Measure the resistance of the cold water inlet sensor at the main control board connector. | Resistance should match the temperature/resistance chart in the Navien service manual. For example, at 77°F (25°C), resistance should be approximately 10 kOhms. |
| Sensor resistance is out of range or open circuit | Inspect the wiring harness and connector for the cold water inlet sensor for damage or corrosion. | Wiring and connector are intact and free from corrosion. |
| Wiring appears OK | Check the voltage at the main control board connector for the cold water inlet sensor. | Typically 5V DC. Refer to the Navien service manual for the exact voltage. |
| Voltage is present but sensor reading is still incorrect | Replace the cold water inlet sensor. | Error code clears and unit operates normally. |
Applicable Navien Models
Source Documentation
Referenced in 2 official Navien technical manuals.
| Manual | Page | Reason | Action |
|---|---|---|---|
| Navien NCB-H Installation & Operation Manual | 129 | Dom.Cold Water Inlet Sensor | Contact technical support at 1-800-519-8794. |
| Navien NFC-H Installation & Operation Manual | 127 | Cold water inlet: thermistor open or short circuit | Auto RESET |
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Cold Water Inlet Sensor | 30010684A | — | Find Part |
| Main Control Board | 30014031A | — | Find Part |
Related Codes on Same Models
Other codes that affect the same Navien model families as E421.
| Code | Description | Severity |
|---|---|---|
| E001 | Ignition Failure | Warning |
| E003 | Abnormal Ignition | Warning |
| E004 | False Flame Detection | Warning |
| E012 | Flame Loss | Warning |
| E016 | Overheating | Warning |
| E030 | Exhaust Overheat Sensor | Warning |
| E031 | Inlet Thermistor Open/Short | Warning |
| E046 | Outlet Thermistor Open/Short | Warning |
| E047 | DHW Thermistor Open/Short | Warning |
| E060 | Fan Motor Abnormality | Warning |