2003 Ford Escape Check Engine Code P0136

Bought car for my son from car dealer. Immediately following transaction, check engine light went on. Had them diagnose. Two codes. Had a misfire (cylinder 5) and a O2 sensor fault (Bank 1 Sensor 2). Fixed the cylinder with new injector, coil and plug. Car ran MUCH better. However, got the O2 fault again. Found worn wire on sensor, so replaced. Got same fault again. Checked up and downstream voltages. Good. Shows cat is working. No leaks found, no other codes. Any ideas of how to clear this code as we can't pass inspection.