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If it were the head gasket you would see signs of smoke in the exhaust, cooling problems, coolant in the oil, coolant loss or loss of compression. So do a compression test and depending on the outcome you will be sure it is not the head gasket. However, It is most likely the intake gasket or something downstream in the exhaust before the O2 sensor. But, since it is a lean bank code, it has to be a vacuum leak before combustion, in most cases, so it is probably the intake gasket.