My 1997 Ford Explorer, SOHC V6 has been posting the same codes for months. I clear the 174 (System to Lean Bank 2), 301 (Cyl 1 Misfire) and 306 (Cyl 6 Misfire) and they return within hours. I have replaced some parts but to no avail... all spark plugs, all plug wires and the ignition coil...