O2 Sensor Code on my 2000 323CI. Please Read.

I have a 2000 323CI and a month ago the check engine light came on. I ran the codes and it gave me a P00130 and P00150 which are "Malfunctioning Left and Right Oxygen Sensors". I erased the codes and the check engine light stayed off for a month but has recently came back on. I erased the codes but it came on within a couple km's again. I think it is HIGHLY unlikely that both O2 Sensors have malfunctioned at the same time so i was hoping for some suggestions on what could be wrong.

A little history:
-September first the car received an Inspection 2 (minus the fuel filter that i am changing this week). (New plugs, all filters, oil, etc.)
-Car runs GREAT and smooth, starts great.
-Gets good gas mileage (7.4L/100km or 32 mpg) (80% hwy/20%city)
-Doesn't use coolant or oil.
-I only ever fun Premium Gas.


Oxygen Sensors can go out at the same time.

I would suspect a vacuum leak first. Visual checks of your vacuum lines / hoses helps, but a smoke test will definitely find a leak.