1998 cadillac deville. anti theft light came on. Engine would start and then go dead. Disconnected battery, then reconnected and car would run ok for several weeks. then same problem again. AC, heater, speedometer, gas gauge, etc. would not work after reconnecting battery. but engine would run fine. ran car for over year this way. Then one day 100 miles from home I saw small sliver of smoke coming from speedometer area. drove car home and drove for another week. Then would crank but not run--my battery disconnection remedy did not work. local mechanic put new starter on and charged $600 for replacing starter and other work he had done--checking fuses etc. How he wants to put new speedometer head on $675--then it might work--but also may have to go to cadillac dealership use their computer to reprogram $175--still may not work. Should I just junk???

SOURCE: 2000 Cadillac Deville DTS is in anti- theft mode

you can reset the system by turning key to on position for 5-10 mins and then try starting it but its probably a short or grounding prob try the engine to chassis ground

SOURCE: 1997 cadillac deville anti theft

This might work for most:

Theft Module Re-learn Procedure without a T5O or T6O
During this procedure, the vehicle battery must maintain a minimum of 10.5 volts. If the battery is not fully charged or is weak, a battery charger may be connected.
NOTE The battery charger must NOT be set in 12 volt boost. This will prevent this procedure from working.
1. Cycle the ignition switch to the "Run" position.
2. Wait a minimum of 10 minutes, cycle the ignition switch to the "Off" position.
3. Cycle the ignition switch to the "Run" position.
4. Wait a minimum of 10 minutes, cycle the ignition switch to the "Off" position.
5. Cycle the ignition switch to the "Run" position.
6. Wait a minimum of 10 minutes, cycle the ignition switch to the "Off" position.
7. Verify the repair.

and if not try this:

STARTING DISABLED/DUE TO THEFT SYSTEM/REMOVE IGNITION KEY"
If the Powertrain Control Module (PCM) does not receive the Fuel Enable message it will use the class 2 serial data line (CKT 1807) to tell the Instrument Panel Cluster (IPC) to display the "STARTING DISABLED/DUE TO THEFT SYSTEM/REMOVE IGNITION KEY" message on the Driver Information Center (DIC) . This will inform the driver of why the engine is not starting. When the ignition is turned "OFF", the IPC will start a timer and display messages to inform the driver to wait for the PASS-Key II timer to run out. The messages will read "WAIT 3 MINUTES", "WAIT 2 MINUTES" (after 1 minutes has passed), "WAIT 1 MINUTE", and "START CAR".
If the timer was not run out, the IPC will not send the Fuel Enable message. When the PCM does not receive the Fuel Enable message, it will command the IPC to re-start the message process.
This message is most likely caused by an improper key resistor value or the IPC did not properly sense the ignition key resistance.
good luck!

SOURCE: '97 Cadllac Deville Concours reading "Anti Theft System Problem"

make sure your key is clean try again, if not the system will tell you to wait 3 mins while key is in ignition this will reset the system and dash will tell you to start vehicle, you should be fine after that, common cause dirty key!!!!!!!!

SOURCE: 1996 cadillac deville anti theft

i had the same prob. i read in the book that came with the car to wipe the key off and try again... such a simple answer, but it worked for me. I always use the trial and error method, lol

SOURCE: Engine and electronics die on ecceleration and remain dead for awhile

ALL Grounds Need to Be Checks on this Including BATTERY They MUST Be CLEAN this Sounds Like a Poor Ground Problem.