Here's a breakdown of how you might approach it:
* Check Engine Light (CEL): If the Check Engine Light is on, that indicates a stored code. However, *simply having the light on doesn't tell you the code*. The 1990 DeVille likely doesn't have a simple "blink code" system accessible via the CEL like some later vehicles.
* Paperclip Method (Unlikely but Possible): Some older vehicles used a "paperclip trick" to access diagnostic codes by shorting specific pins in the car's diagnostic connector (usually located under the dash). This method is unreliable for several reasons, including the possibility of damaging the car's computer and it simply might not work at all. I strongly advise against trying this without a detailed wiring diagram for your specific year and model because you could cause damage.
* Dedicated Scan Tool: The most reliable method is using an OBD-I (On-Board Diagnostics I) scan tool. Since your car is from 1990, it uses the older OBD-I standard, which is different from the OBD-II standard used in most cars from 1996 onward. You'll need a scan tool that's compatible with OBD-I. These tools can be found online (eBay, Amazon) or from automotive parts stores. Some auto parts stores may even read the codes for you for free or a small fee.
In summary: The paperclip method is a long shot and risky. Your best bet is to either use a scan tool specifically designed for OBD-I systems (for a 1990 Cadillac) or visit a mechanic or auto parts store that can retrieve the codes for you. Don't attempt the paperclip method unless you are extremely knowledgeable about automotive electronics and have a wiring diagram.
