NR-240A-NG // Low pressure output, not heating, Error Code 38E

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JM2020

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Good Day Experts,

I am experiencing low flow and cold water from my NR-240A-NG tankless water heater.

I have cleaned the air intake filter (light skim of dust/derbis), drained the water heater, cleaned the cold water inlet filter (it is now somewhat broken/screen in pieces), dirt/mud trap (this was filthy with dirt and brown water) and the recirculation inlet filter.
Occasionally I am seeing Error Code 38E (Abnormal circulation pump).

My questions
- Will the broken inlet filter cause any of these issues? I will pick a new one up in the city tomorrow or order one if I cannot find one locally.
- Will cleaning the scale/build up with vinegar or a mild cleaning solution fix this issue? I have yet to do this but will get a submersible sump pump tomorrow and try cleaning this.
- Error Code 38E also mentioned it could be a faulty flow sensor valve, I just replaced this 2-3 months ago so I am leaning against this being the issue. Is there a way to verify/test this with a multimeter or some method? What voltage or resistance should I see through this?
- If it is in fact the circulation pump, how can I verify this before replacing? Multimeter test? Some other test?
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Any help or guidance would be greatly appreciated. I would consider myself quite handy and able to perform any troubleshooting given I have the appropriate tooling. I am also hoping to connect with Navien customer support tomorrow, but of course, these issues always arise after business hours.

Thanks
 

JM2020

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Update - The circulation pump does get 120V when it is turned on. It does however make a strange sound when it is turned on that I don't recall it making before.
 
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