Are you sure the 5v your getting at the O2 sensor isn't just the supply voltage from the ECU, as these run at 5v !
Get the faults/"P" codes read in the ecu, if it says the oxegen sensor is open circit/short to positive change the sensor,
give it a good thrash (low gears, high rpm) down the road for 5-10 miles and take it back for a retest.
Unburnt fuel in the cat will wreck it in a very short period of time.
