2005 Buick Lesabre has error code P0300-P0305

p0300 means that the the car's computer has detected that not all of the engine's cylinders are firing properly. A P0305 code means that the the car's computer has detected that one of the engine's cylinders is not firing properly. you can get a code scan tool to clear the fault codes.

This is a misfire condition that usually indicates faulty spark plugs or a bad ignition coil. Other option is bad timing but, change the plugs and that coil and your problem may resolve.