421Error - Navien NPE-180A Service Manual

5.1.12 421Error

Error conditions and Check Items
Error
E421
Heat exchanger input If an error (open: 14°F (-10°C) or lower) in the heat exchanger input temperature sensor is detected, the system
temperature sensor displays the 421E error on the front panel. If this occurs, the water heater initiates shutdown.
open
1. Check if the heat exchanger input temperature sensor connector is wet due to any reason, including leakage.
Check items 2. Replace the defected heat exchanger input temperature sensor.
3. Replace the controller.
Check method
Fault
Defective sensor
Possible Issues
100 NPE Service Manual
Is the inlet thermistor
normal?
Yes
Check connector
(#1 and #2 of CON 7)
Possible Causes
Defective temperature
Check if the temperature sensor is open and if the connector is connected properly.
sensor connector
Check the resistance of the temperature sensor. (Defective if it is 30kΩ or higher)
Temperature sensor Replace the temperature sensor if the resistance value is abnormal.
●
Check the temperature displayed on the front panel.
●
Defective PCB If the issues continue despite checking the items above, replace the PCB.
Heat exchanger input temperature sensor /
connector
Description
NO
Replace inlet thermistor
Resistance between #1 and #2 of CON7 is from
0.437kΩ to 176kΩ at normal condition.
(see page 52 for Wiring Diagram)
Before measuring resistance of the thermistor,
detach the connector from the PCB.
Check method
Check if the secondary water temperature
Error type : MΩ Open
sensor is open
Version 2.21