Umm, it's hard to make suggestions without seeing it but I'll try anyway and if it's not showing up with any codes chances are it's NOT a sensor.

It could be something simple as a leak in your air intake or cold air is getting into the exhaust some how, so a leak there too.

This does does sound like you have some sort of exhaust type of problem.

In my opionon you should try the cheapest simpliest solution and then work your way up.