u/Fickle-Tea-7847

Long story short, for the last two months these codes have come and gone multiple times. My mechanic is dead set on the issue being caused by the lack of a muffler on my F150. However, it has not had the majority of the exhaust pipe (cut off just behind the passenger door) for the last 4 years and it has never been an issue. Mechanic added 3ft of pipe and the codes went away for a week. Where on earth do I start? Not noticing any significant drivability issues but it's definitely a little sluggish.