I just finished reassembling after a head gasket job and am getting those codes. At first I had a p156 along with the 171/174 which is o2, bank 2, but after a day or two of driving around that code went away. At first I was thinking vacuum leak, however I am having no symptoms of a leak. The car does not stumble or bog down on acceleration, nor does it idle rough. I read that a bad O2 sensor can cause a 171/174 code, is this true? Should I still change out the O2 sensor even though the code disappeared? Any help is appreciated.