02 sensor question

[Ray Sta.Juana]

Hey guys, I don't know if you remember but I had a problem with my exhaust
manifold about a month ago and it broke right around where it bends on the
secondary under the car.  I got it fixed with a primary and secondary from a
B13 but my question is this, I've been getting worse gas mileage since then
and I searched the archives to see what I could do about it and I read that
I should replace my 02 sensor. Well, since I drove around with a really bad
exhaust leak for a couple of weeks, could both my sensors be bad or is there
a way to test them? Could it be the sensors or should I have gotten the
parts off a 98 Sentra SE instead of a B13? I average my mpg (using the same
gas station, averaging mileage, etc.) at about 19-21 mpg on a 90K car. I'm
hoping it's a simple fix but with my luck it's probably not. Thanx in
advance for any help.

Ray S
98 Sentra SE
San Bernardino