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I need the brand and model number for your unit
Give a few while I pull your manual
Ok, a 12 error is a generic flame failure error. Correcting this issue is usually a job for a Rinnai trained pro. Are you using this for space heating or just DHW (domestic hot water)?
It's possible the exhaust could be clogged with frozen condensation. That's about the only weather related cause for this type of fault. Is it attempting to fire up? you get ignition then it shuts down?
That was 2 questions. Yes to both?
Ok the you have a flame rectification issue. If you've cleaned the sensor rod the next step would be to replace it. I assume you checked the connections
Here's the troubleshooting guide. It goes into a lot more detail than I can here.