The PCM will set the P0172 code when it can no longer adjust the air/fuel ratio to the desired amount. Specifically, because there is an excess amount of fuel.

Q: Code P0172 came up in my 2010 Chevy Malibu. The dealer charged me $100 to troubleshoot the code. I was told that all the sensors were good and to run the gas tank low and fill up with high ...