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A 12 error is a flame failure error. The only time it's likely to be a gas problem is with a new install (which is why it's suggested in the manual). In an older unit it's much more likely to be an ignition or flame sensing problem. If the unit hasn't been serviced in a while it would be a good idea to have a pro check it out and clean out the combustion chamber.
No problem. Drainage issues have nothing to do with your water heater. There are a few things that can cause temp fluctuations but they may or may not be related to the 12 error. You really need to have a pro check out and service your heater. The problems will only get worse over time unless you get the heater serviced.