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A 3C code refers to a bad connection to the fan or a faulty fan.
Here is a blog on the subject.
talked to buderus and told them my codes and problem, they where familiar with the issue and said it was a bad tachometer in the combustion motor. Grabbed the last motor around and im up and running. Also i was able to run the heat by pulling the 4 prong plug on the blower motor when the ignitor was on, this causes the fan to speed up an down, then just put the plug back in and it will light and run normal until the call for heat ends and will trip on the next call, but it beats having no heat.
Ohming out the wires back to the 81 pin connector behind the UBA will determine if the wiring harness needs to be replaced or the fan itself.
here is a link to service Tip #3