F12 occurs on a multi-system or quick connect. It means one of the units on the system suffered a flame loss. More specifically, it means the flame rod stopped detecting flame on the burner. The most typical causes are as follows:
1) insufficient BTUS (low gas pressure)
2) leaking heat exchanger
3) clog in the manifold, burner, or heat exchanger
4) stuck flue damper or improperly sized venting
5) dirty flame rod
6) damaged circuit board or gas valve
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