It's possible that the wiring to the second oxygen sensor - the one downstream of the catalytic converter - is damaged or that the sensor itself has gone bad.

Unless someone put leaded petrol in the tank, your catalytic converter should have a life of at least a decade... so the code 67, which would indicate that the cat isn't cleaning up emissions, could possibly be based on the failure of the second oxygen sensor.

Here's the first step I would take: An emissions test would be able to conclusively prove whether or not the catalytic converter has indeed worn out.

If your emissions are within standard levels (low HC, CO & Nox), then the cat is most likely fine- and the problem lies elsewhere in the emission-detection system.
If the HC, CO and Nox readings are off the chart, then the cat is more than likely crapped out, and will need replacement.

I hope this helps.