Error - Navien NCB500-2S/37K Service Manual

5.2.17 Error 421

Error conditions and Check Items
Error
E421
Cold water inlet thermistor open or short
Check items
86 Troubleshooting
If an error (open: -10°C or lower) in the DHW Outlet Elbow input temperature sensor is
detected, the system displays the 421E error on the front panel.
1. Check if the cold water temperature sensor connector is wet due to any reason and if
the connector is connected properly.
2. Check if the temperature sensor is open or short.
Is the inlet thermistor
normal?
Yes
Check connector
Description
No
Replace inlet thermistor
See page 38 for Wiring Diagram.
Before measuring resistance of the
thermistor, detach the connector
from the PCB.