Fuel system evaporation lines

Most definitely. This would be a little different from a vacuum leak, but a leak in the evap system. The cars on-board computer monitors for leaks in the evap lines and sets a trouble code if one is found. Sometimes it can also come up if you don't tighten the gas cap. Good luck with your repairs.

Thanks Luke. The mechanic wants to replace the catalytic converter - $$$

try a smoke test to find the leak

thanks Andrew - great idea.