421Error - Navien NCB-180 Service Manual

5.2.17 421Error

Error conditions and Check Items
Error
E421
Cold water inlet
thermistor open or
short
Check items
110 NCB Service Manual
If an error (open: 14°F (-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
(#1 and #2 of CN 13)
Description
NO
Replace inlet thermistor
Resistance between #1 and #2 of CN13 is from
0.437kΩ to 176kΩ at normal condition.
(see page 56 for Wiring Diagram)
Before measuring resistance of the thermistor,
detach the connector from the PCB.
Version 1.0