O2 Sensor-2001 Galant-HELP!!

Ok, here's the story. My service engine light came on and the car
started hesitating when I would accelerate. Had a mechanic read the
code and he said my pre-cat O2 sensor is shot. Replaced the part myself
and drove about 120 miles the next day. Car was running great and no
engine light. The next day I start the car and the engine light is back
along with the sluggish performance. 72,000 miles on the car.

Any ideas folks?? What would be causing the light to keep coming on?? I
can't believe a brand new sensor would be the problem.

Thanks in advance..