* Incorrect key: The most common cause. Make absolutely sure you're using the correct key, specifically the key that came with the car or a properly programmed replacement. A worn key might not work reliably.
* Low battery: A weak battery in the car can cause intermittent electrical problems, including anti-theft system malfunctions. Try jump-starting the car with a fresh battery.
* Faulty Passlock system: The 1996 Seville likely uses a Passlock system which relies on a resistor in the key to communicate with the vehicle's computer. If this system is malfunctioning (either in the key or the car's receiver), it will prevent the car from starting. This often requires professional attention.
* Alarm triggered: The alarm might have been accidentally triggered. Check for any obvious causes and disarm it using the key fob, if equipped.
* Security system malfunction: The system itself could have a fault. This requires diagnostic tools and potentially repair or replacement of components.
What to do:
1. Try the spare key: If you have a spare key, try that first. This eliminates the key as the problem.
2. Check the battery: Make sure the car battery is strong and properly connected.
3. Check the alarm system: If the car has a separate alarm system (not just the Passlock), ensure it's not triggered.
4. Consult a professional: If none of the above works, you'll need to take the car to a qualified mechanic or locksmith specializing in older Cadillacs. They have diagnostic tools to check the Passlock system and other security components. They may be able to reprogram the system if needed, though this is often costly.
Important Note: Attempting to bypass or disable the anti-theft system without proper knowledge can damage the vehicle's electrical system. It's always best to seek professional help if you are having persistent issues.
