421Error - Navien NFC-175 Service Manual

5.2.22 421Error

Error Condition and Check Items
Error
E421
Check Items
Version 1.0
Abnormal Cold water intake sensor
1. Check for disconnection in controller harness.
Check for disconnections between harness connecting controller and water intake
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temperature sensor.
2. Change water intake temperature sensor.
If the problem persists after completing (1), close external water intake supply valve and
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purge water pipes of residual water. Change water intake temperature sensor.
3. Change PCB
If the problem persists after completing all the actions above, change the PCB.
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Description
water intake sensor
NFC Service Manual 107