Cylinder misfires are usually due to 3 possible causes. Spark plugs need replaced, fuel injector clogging up, or spark plug coil failure. If the plugs have been changed recently, the cause is probably the spark plug coil going bad. http://images.search.yahoo.com/images/view;_ylt=A0PDoTEvd0ZPtg0AgDmJzbkF;_ylu=X3oDMTBlMTQ4cGxyBHNlYwNzcgRzbGsDaW1n?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3D2005%2Bcadillac%2Bcts%2Bfiring%2Border%2Bdiagram%26fr%3Dyfp-t-701%26fr2%3Dpiv-web%26tab%3Dorganic%26ri%3D0&w=260&h=250&imgurl=allinfoaboutautorepairs.com%2Fuploads%2Fimages%2Ffiring%2520orders%2F110544534.gif&rurl=http%3A%2F%2Ffiringorders.allinfoaboutautorepairs.com%2Findex.php%3Fpage%3D30878&size=4.5+KB&name=All+Info+About+Auto+Repair%3A+2005+Cadillac+CTS+3.6+liter+V-6+VIN+%26quot%3B7%26quot%3B&p=2005+cadillac+cts+firing+order+diagram&oid=98cfb54b7bacec81c084d5f914469c14&fr2=piv-web&fr=yfp-t-701&rw=2005+cadillac+cts+firing+order&tt=All%2BInfo%2BAbout%2BAuto%2BRepair%253A%2B2005%2BCadillac%2BCTS%2B3.6%2Bliter%2BV-6%2BVIN%2B%2526quot%253B7%2526quot%253B&b=0&ni=48&no=0&tab=organic&ts=&sigr=124als7f2&sigb=13v8r78d6&sigi=128phrnh1&.crumb=qe4L1J9ejGj you can do this yourself easy to replace http://www.autozone.com/autozone/parts/Duralast-Ignition-Coil/2005-Cadillac-CTS/_/N-jmlqeZ9n80r?itemIdentifier=201817_0_0_