2003 Cadillac STS Checking and clearing "Check Engin&q

The most friendly, and cost efficient would be the on-board system that you vehicle is equipped with. If you press the 'info' button up, and 'on/off' at the same time and hold them, it will enter a segment check where all displays illuminate. After that it should display ALL? In the info center. If you press info up or down it will scroll each module alphabetically, or if you press on/off it will say ALL CODES, and pressing on/off again will display all codes for all modules. If after it displays ALL CODES, you were to press info up or down it would display ALL CLEAR CODES, and pressing on/off would clear all codes. When after entering diagnostic mode, info scrolls modules, and on/off executes the command. It will not tell you the code description, just the numbers(e.G. PXXXX, BXXXX, CXXXX, or UXXXX).