I just had my 1997 Cadillac Seville STS have a senior momentin the last 3 weeks.

I went to start the car and it appeared the car was in "theft" mode and no attempts by me with the key fob, key in the door or radio button access to OBD II computer could get it out of this mode. The honking horn at 7 AM was lovely but the key in the door eventually shut off the horn.

The dealer started withdrawling cash to the tune of $150 worth of worthless diagnosis and I told them to stop. They suspected the BCM. I suspected they had a goober mechanic on the job.

I bought a BCM from a junkyard for $40 and went to install to BCM but the battery had died and it was new. My battery charger would not recognise this dead battery so I put a freshly charged battery in parallel with jumper cables and fast charged both batteries for a few hours.

Eventually, like the terminator, my Caddy came back to life and I turned the key and she started and amazingly the dash worked and I could get into OBD II mode.I had too many to count "errors" on all the systems soI spent 30 minutes clearing all the codes while the car ran and came up to temperatureand further charged the battery.

What I think happened is the "theft mode" set a bit in some computer that could not be "unset" and when my battery went dead for 3 weeks the stored values all drained and went back to factory defaults permitting me to start the car,

Any suggestions or theories? Anyone need a 1997 BCM?