You said the car runs fine, means this is a O2 problem. The O2 is only being used in close loop. This will throw the code only in closed loop. So it will throw the code during cruising at a steady speed or at idle. You can hook up a multimeter to measure the voltage of the O2. Red to white wire of o2 and black to any ground. When idling the O2 should sweep from .2 volts to .8 volts and repeating this over and over. Check this also at pinout D14 of ecu too. If voltage does not move up and down you have a bad o2. Hope this helps.