100 NPE Service Manual
5.1.12 421Error
Error conditions and Check Items
Error Description
E421
Heat exchanger input
temperature sensor
open
If an error (open: 14°F (-10°C) or lower) in the heat exchanger input temperature sensor is detected, the system
displays the 421E error on the front panel. If this occurs, the water heater initiates shutdown.
Check items
1. Check if the heat exchanger input temperature sensor connector is wet due to any reason, including leakage.
2. Replace the defected heat exchanger input temperature sensor.
3. Replace the controller.
Before measuring resistance of the thermistor,
detach the connector from the PCB.
Check connector
(#1 and #2 of CON 7)
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)
NO
Is the inlet thermistor
normal?
Yes
Check method
Fault Possible Causes Check method
Defective sensor
Defective temperature
sensor connector
Check if the temperature sensor is open and if the connector is connected properly.
Temperature sensor
Check the resistance of the temperature sensor. (Defective if it is 30kΩ or higher)
●
Replace the temperature sensor if the resistance value is abnormal.
●
Check the temperature displayed on the front panel.
Possible Issues
Defective PCB If the issues continue despite checking the items above, replace the PCB.
Heat exchanger input temperature sensor /
connector
Check if the secondary water temperature
sensor is open
Error type : M Open
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