I own a 1997 Cadillac STS w/126000 miles. As I write this, the "check engine light" is on and stays on. According to the manual this means a problem exists with the emmision system. Instead of getting a diagnostic done, I would like to replace the O2 sensors and see if that rectifies the problem. My actual question is where are the sensors located on this car and are there any special tools involved in replacing them.