ck ng light Autozone said oxygen sensor & sensor in trans is this exps to fix on 2003 Cadillac Seville

my mechanic said the sensor in trans is internal and he does not do this the oxygen sensor also needs to be replaced how expensive is thks type of repair

Asked by for the 2003 Cadillac Seville
It really needs to be diagnosed to make sure the sensors are the problem or something else causing the codes.