421Error - Navien NCB-24LSWE Service Manual

5.2.18 421Error

Error conditions and Check Items
Error
E421
Cold water inlet
thermistor open or
short
Check items
Version 1.0
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
Description
NCB Service Manual 113