Navien NCB-180, Service Manual

Page: 113 / 149

110 NCB Service Manual Version 1.0
5.2.17 421Error
Error conditions and Check Items
Error Description
E421
Cold water inlet
thermistor open or
short
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.
Check items
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?
Resistance between #1 and #2 of CN13 is from
0.437k
Ω
to 176k
Ω
at normal condition.
(see page 56 for Wiring Diagram)
Check connector
(#1 and #2 of CN 13)
Replace inlet thermistor
Before measuring resistance of the thermistor,
detach the connector from the PCB.
NO
Yes