Question

2002 Cadillac Deville 4.6L with code P0303, the car is running rough and has bad acceleration

Answer 1

P0303 Misfire number 3 cylinder

Whenever there is a misfire it means that one of the cylinder or more  is not contributing , Some misfire can be from a mechanical condition where cylinder head is a fault , and  a leak down test could be used to  help identify if the cylinder head was at fault, There is also a possible ignition misfire ,The ignition system uses a ignition coil cassette, The ignition coil for this cylinder can be at fault