Navien E421
Navien E421: cold water inlet sensor fault
Short answer
E421 means the cold water inlet thermistor, the sensor that measures incoming water temperature, is reading as an open or short circuit. The heater uses this reading to calculate how much heat to add. A technician checks the wiring and replaces the sensor if needed.
In this guide
To hit your set temperature quickly, a tankless heater needs to know how cold the incoming water is. The cold water inlet thermistor provides that reading. E421 means the sensor's circuit reads as open or shorted.
Some Navien models have a second inlet sensor, which reports its own code (E432) if it fails.
The meaning below comes from Navien's manuals for its NPE condensing tankless water heaters. Many codes are shared with Navien combi boilers, but check the manual for your model if something does not match.
For this code, Navien's user manual directs owners to contact Navien technical support (1-800-519-8794) or a licensed installer.
Common causes
- A loose connector or damaged wire
- Water or corrosion at the connector
- A failed sensor
Inlet sensors sit on the cold water line where they can be exposed to freezing in unheated spaces. If the code appeared after a cold snap, mention it to your technician, and check that freeze protection is working.
The repair
After checking the wiring and measuring the sensor, a technician replaces it if it reads open or shorted. It is a small, inexpensive part.
How to reset a Navien
- Close all hot water taps.
- Press the power button on the front panel to turn the unit off, wait about a minute, then turn it back on.
- Open a hot tap and watch whether the code returns.
See every code on our Navien error codes page.
Common questions
What is the difference between Navien E421 and E432?
Both are cold water inlet sensor faults. E421 is inlet sensor 1 and E432 is inlet sensor 2, on models that have two.
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Other Navien codes
| Code | Meaning |
|---|---|
| E003 | Ignition failure |
| E004 | False flame detection |
| E012 | Flame loss |
| E016 | Heat exchanger overheat |
| E030 | Abnormal exhaust temperature |
| E031 | Burner overheating |
| E046 | Limit control (high limit switch) abnormal |
| E047 | Exhaust temperature sensor (thermistor) open or short |
| E060 | Dual venturi abnormal operation |
| E109 | Fan motor abnormal operation |
| E110 | Exhaust blockage (condensate blockage) |
| E407 | Hot water outlet temperature sensor open or short |
| E421 | Cold water inlet temperature sensor open or short |
| E432 | Cold water inlet 2 sensor open or short |
| E434 | Water adjustment valve fault |
| E438 | Circulation pump fault |
| E439 | Flow sensor fault |
| E441 | Second hot water temperature sensor open or short |
| E445 | Bypass mixing valve fault |
| E515 | Control board (PCB) abnormal operation |
| E517 | DIP switch setting fault |
| E593 | Front panel key fault |
| E594 | EEPROM (memory) fault |
| E615 | Input and memory fault |
| E736 | Cascade communication fault |
| E740 | Outdoor temperature sensor fault |
| E760 | Flushing or service alarm |
| E762 | Vent pipe not properly inserted |
| E773 | Using the wrong gas |
| E782 | Main panel communication fault |
| E785 | Flow switch or flow sensor fault (with AHU) |
| E788 | Gas type mismatch between board DIP switch and front panel |