went with my son to check out a 2004 dhs for sale.Now he is looking for a work car to save wear and tear on his new truck.He has hip problems and he loves the heated seat option and the roominess and the fuel milage of the Deville.When we started the car the engine was missing badly and check engine light was on.we checked the codes and we got four different codes relating to front and rear o2 sensors plus a p0300 code for multiple misfires.HE has the code numbers written down and I cannot remember them now.I know that they had just recently changed starter and had trouble with the starter not working afterwards due to not hitching up ground wire.My question is does anyone have a good guess as to why ,after a starter change ,would the engine run rough at idle and set multiple o2 sensor codes and the misfire codeKeep in mind that these were not professional mechanics who did the work.I know I should have all the code numbers and I can get them later but does anyone want to take a guess with this limited info-thanks