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Navien water heater error E421
Inlet temperature sensor fault. The cold water inlet thermistor has failed.
Moderate · needs a technician
E421
Meaning: Inlet temperature sensor fault
What causes E421
The cold water inlet thermistor has failed.
What to do
The sensor needs replacing.
Can I fix it myself?
Not really. The part involved has to be tested with a meter and usually replaced.
If the code comes back
Book a visit and add the code to the booking. A technician arrives knowing which parts to bring, diagnoses the water heater for a flat $89 and gives you an exact quote before doing any work.
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