My daughters car (2006 Saturn Ion Level 2 - 108,000 miles) starting running rough for a while but then would run fine. I pulled the codes and got a P0326 so I replaced the knock sensor. Next day same problem, pulled the codes again and got a misfire code. Replaced the coil pack and it runs fine. EXCEPT now then Check Engine light comes on and I get P0420 (Cat Efficiency Below Threshold), P0326 (Knock Sensor Range/ Performance) and P0106 (MAP Range/Performance). I am beginning to think there is a power and/or ground issue. Any thoughts where I can start looking?