A code 63 refers to the oxygen sensor immediately downstream of the catalytic converter, threaded into the cat itself.

One of two things are happening.
a) The ECU isn't receiving any voltage signal from the sensor (either due to a broken lead or insufficient ground).

b) The voltage coming back from the sensor is deemed by the ECU to be outside of normal parameters.

If you still have the original CD stereo in the car, find the anti-theft code.
Once you've got it, disconnect the positive battery lead for 10 minutes to clear the code from the ECU.

If there is any doubt in your mind about the connection between the sensor and the wiring harness, check it to make sure the sensor is plugged in before re-connecting the battery lead.

Once that's done, go for a drive long enough to get the catalytic converter good and hot. The code shouldn't return.