Unusual Navien 003E Code + Solution

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Boston, MA
Hi Everyone,
I have just worked through a 003 error code, which was not supported by the Service manual. I could not find any record of this online so I thought I'd share in case anyone else sees the same problem.

The point - the 150V supply from the transformer will mean the flame rod current is too low to detect and will trigger the 003E code. This is not referenced in the Navien service manual.

The story - I had a small but growing leak in the flow sensor of a Navien CH-240-ASME boiler. I removed the old one and replaced it with a new one, fixing that problem. On startup I received a 003E. I found the service manual and checked out the flame rod. This had not been replaced in 11 years so was fairly burnt out. I replaced it but it did not fix the problem. I checked the current coming through the flame rod and it was measuring 0.2-0.5 µA - much lower than expected. The next target in the service manual was to replace the PCB, which I did but this did not help either. The unit seemed fine in every way, except the flame rod current. I eventually went through and measured every voltage and found the 150V supply was missing from the transformer. I could not find a reference in the Navien manuals but it seems the only purpose of this voltage is the flame rod AC input. Once I switched out the transformer I got back to a current of about 5 µA and the code cleared. The old transformer appears to have corroded by being in proximity to the original flow sensor assembly leak.

I hope someone finds this helpful and can avoid buying an unnecessary PCB.
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