2003 Cadillac Deville check engine light

If it flashes, that indicates a catalyst damaging misfire, bad fuel is but one of the possibilities. Does it misfire/run rough. Does this misfire occur; a) at all times, b)under load, c) cold and hot engine? You can check your own codes by pressing info up, and on/off with the key on. After a segment check it will display ALL? on the info center. From here press the info button up, or down, to locate PCM? and then press on/off. Use the info button to navigate to PCM CODES?, and then press on/off. Recored the codes, and reply to post. I would guess you have at least a P0300 misfire detected, but there may also be a cylinder specific(P0301, 302, etc...) misfire code that will narrow it down. It could be a coil, plug, injector, or mechanical condition. Since it is flashing, I would speculate that it is a hard misfire that occurs continuously. If you can find which cylinder to focus on, you can inspect the plug for that cylinder, and the coils can be swapped bank to bank to see if the code follows a coil.