1998 Cadillac Deville runs good but???

has the fuel filter been changed? on the temperature light. when does it come on when you start the car or after it has run for a while? check the sending unit for the temp. disconnect it at the sending unit to see if it could be the dending unit. if the light goes out it would be the sending unit. then touch the wire that goes to the sending unit grounding it and check the light. it should come on, check the fluid, make sure it is circulating.

you will have to reset the computer using the onboard diagnostics system a lot of times just disconnecting the battery will not do it that car has a onboard diagnostics system and you can go in and clear the codes to get started you need to have key on , engine not running push warm and off button at sdame time and then you should be able to go through the diagnostics of all the systems, the owners manual will tell you how to do it i think as well you should be able to walk yourself through it if it dont help you let me know i have the menu for it and i can get it to you some how