To find out what "E31" means, you'd need to consult:
* The owner's manual: It might have a section on troubleshooting codes, especially if the car has a simpler onboard diagnostic system.
* A Cadillac DeVille service manual specific to the 1992 model year: These manuals are often detailed and provide code descriptions and troubleshooting steps. You can find these online (e.g., eBay, Helms, etc.) or at auto parts stores.
* A professional mechanic: They have access to diagnostic tools and service information that can pinpoint the problem.
The code might be specific to a particular aftermarket diagnostic tool or even be a misreading of another code. Without more information about the source of this error code, it's impossible to interpret it.