1999 Expedition, 5.4 engine, 170,000 miles.
While driving, started running rough.
No CEL, but Pending code P0304 (Cylinder 4 misfilre).
Replaced #4 COP and plug.
Still running rough and after driving a while, the CEL illuminated. Displayed P0304 DTC and P0136 (and O2 sensor circuit Bank 1...