Pass key fault indication light

Not sure what year you have, but you might try leaving the key in the "ON" position for about 15 minutes and see if the security light goes off, if it does it should start for you. The lock cylinder halo will need changing fi it works.

The Security Indicator Light illuminates to indicate that the security system is either working properly or to let you know that there is a fault in the system.
If the Security Indicator Light is flashing with the key out of the ignition lock and accessory position, this is a normal sign that the alarm and the immobilizer system is active.
While starting the car or key in the "run" position, the security lamp should illuminate briefly then go out. This a "bulb check" and is also normal.
If the Security Indicator Light is on steady with the key in the "run" position, and the vehicle won't start, there could be a fault with the system. In this case, the first thing to try is a different key to see if this starts the vehicle. If it starts, the key is faulty or the system has a problem with that key. If you are still getting a no-start with the light on steady, the system is at fault and will need to be inspected.

Anti-theft system transceiver ?
Theft Deterrent System
The Theft Deterrent System on this vehicle is comprised of two separate systems, the Personal Automotive Security System (PASS-Key?® III) and the Content Theft Deterrent (CTD) system. These system are described and diagnosed separately within this section. For CTD operation, refer to Content Theft Deterrent (CTD) Description .
SECURITY Indicator The yellow indicator SECURITY on the instrument cluster is controlled by both the PASS-Key?® III system (VTD module) and the CTD system (RIM).
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PASS-Key?® III system, the VTD commands the instrument cluster to control the indicator only when the ignition switch is ON.

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CTD system, the RIM commands the instrument cluster to control the indicator only when the ignition switch is OFF.
PASS-Key?® III System The PASS-Key?® III system is designed to help prevent vehicle theft by disabling the engine unless an ignition key with a specific electrical value is used in the ignition lock cylinder. The PASS-Key?® III system operates independently of the CTD system. Protection is provided even if the CTD system is not armed. The components of the system are the ignition keys (black or gray), ignition lock cylinder, Theft Deterrent (VTD) module, Reader Exciter, Powertrain Control Module (PCM) and two starter relays (START 1 and 2) in the underhood fuse block.

The pass key light or lock picture/icon is a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM (Theft Deterrent Module). These 3 parts are in CONSTANT communication. If the key is in the ignition, then these parts are sending signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, the car misinterprets this as an attempt at theft and goes into antitheft mode. The car thinks its being stolen or hotwired because it is not getting that signal from YOUR specific key. This is why the security light turns on and shuts down your vehicle. This link explains it pretty well.
http://vatspasslockpasskeysecurityhelp.yolasite.com/

Get a second ignition key, the one you are using is dirty or moisted, you need to turn on and off the car witha clean key, hapens to me once in a while.!!

Ya, this is a common and potentially expensive problem on GM vehicles like your cadillac. Go to this link for a rundown of how the system works and ways to fix it yourself

http://vatspasslockpasskeysecurityhelp.yolasite.com/

This is code 12 and it means "System Normal"
Here is the complete list of fault codes for your Buick
GM common codes for OBD1 (for vehicles made before 1995)
12 System normal
13 oxygen sensor circuit open
14 coolant sensor high resistance or shorted
15 coolant sensor circuit low or open
16 direct ignition system (DIS) fault in circuit
17 cam position sensor fault
18 crank or cam sensor error
19 crank sensor circuit fault
21 tps sensor out of range
22 tps sensor signal voltage low
23 intake air temp sensor out of range, low
24 vehicle speed (vss) sensor circuit fault
25 air temp sensor sensor out of range, high
26 quad-driver module (computer) circuit #1 fault
27 quad-driver module (computer) 2nd gear circuit
28 quad-driver module (computer) circuit #2 fault
29 quad driver module (computer) 4th gear circuit
31 cam position sensor fault
32 egr circuit fault
33 map sensor signal out of range, high
34 map sensor signal out of range, low
35 idle air control sensor circuit fault
36 ignition system circuit error
38 brake input circuit fault
39 clutch input circuit fault
41 cam sensor circuit fault, igntion control circuit fault
42 electronic spark timing (EST) circuit grounded
43 knock sensor
or electronic spark control circuit fault
44 oxygen sensor lean exhaust
45 oxygen sensor rich exhaust
46 pass-key II circuit or ps.pressure switch circuit fault
47 pcm-bcm data circuit
48 misfire diagnosis
51 calibration error, mem-cal, ecm or eeprom failure
52 engine oil temperature circuit, low temperature indicated
53 battery voltage error or egr or pass-key II circuit
54 egr system failure or fuel pump circuit low voltage
55 a/d converter error or pcm not grounded or lean fuel or frounded reference voltage
56 quad-driver module #2 circuit
57 boost control problem
58 vehicle anti-theft system fuel enable circuit
61 a/c system performance or degraded oxygen sensor signal
62 engine oil temperature high temperature indicated
63 oxygen sensor right side circuit open or map sensor out of range
64 oxygen sensor right side lean exhaust indicated
65 oxygen sensor right side rich exhaust indicated
66 a/c pressure sensor circuit low pressure
67 a/c pressure sensor circuit or a/c clutch circuit failure
68 a/c compressor relay circuit failure
69 a/c clutch circuit head pressure high
70 a/c refrigerant pressure circuit high
71 a/c evaporator temperature sensor circuit low
72 gear selector switch circuit
73 a/c evaporator temperature circuit high
75 digital egr #1 solenoid error
76 digital egr #2 solenoid error
77 digital egr #3 solenoid error
79 vehicle speed sensor (vss) circuit signal high
80 vehicle speed sensor (vss) circuit signal low
81 brake input circuit fault
82 ignition control (IC) 3X signal error
85 prom error
86 analog/digital ecm error
87 eeprom error
99 power management

My light stays on

i had the same prob in my 94 the ignition cylinder is bad change the ignition and i swear it will work perfect

WELL I HAD THE SAME PROB. ON A 95 CHEVY CAPRICE.I REMOVED THE IGNITION AND THE WIRE WAS BROKE.WHEN U TURN THE KEY THE PASS KEY WIRE TURNS WITH THE IGNITION,WHAT A FAULTY DESIGN.ANY HOW BUY A NEW IGNITION AND KEY THAT SHOULD SOLVE THE PROB. GOOD LUCK