if i am right, the issue is with deville's as well. I would suggest you try the ignition relearn proceedure.....
turn ign. to "on position" (do not turn engine over) and do a one-one thousand count to 10,
turn key to off for 3 sec,
repeat steps 1 and 2 two more times (total of 3 times)
on last turn to off, remove key from ignition, replace and turn over. I hope that works for you.
If not, try this ridiculous idea that DID work for me. I have a 93 deville as well as my 88 eldo. I had the security issue with deville so while sitting at my desk making calls to locksmiths I looked to my right. there was a small nail file my daughter left behind. I turned it to the less rough of sides and lightly ran across the metal portion of the chip on both sides. went to my car and to my surprise, IT FREAKIN WORKED.... i promise you, sometimes the simplist and dumbest ideas actually worked. I verified by taking spare key and before using file on it i tried it, no bueno. filed it about 3-4 times lightly and it worked. I assume it just cleaned the contact points. that was about 3 months ago, still works. last reccomendation, autozone sells a remote start system. i believe you can hook that up, costs about $49. and you can eliminate buying $100 plus keys from dealer. i heard about attching a volt meter to key, get the reading, should be around 80-82 (i think, don't quote me on those numbers), you can then go to electronics outlet and get a small computer board, add resistors with the output, wire the ignition to that board and it will bypass the key chip. if you have heard of this let me know please, i am trying to validate that idea....i wish you the best of luck.
what more info do you need?
Trouble codes retrived with a professional scan tool not just a code reader. A plain code reader will not accesss the right computer.