The OBD2 trouble code B1004 for a 2003 Buick LeSabre indicates an issue with the Driver's Key Switch Circuit. This code typically means there is a problem with the key switch or related wiring. The key switch circuit is responsible for communicating with the vehicle's computer to ensure that the correct key is being used during starting and operation.

To diagnose and resolve this issue, you can try the following steps:
1. Check the key for any damage or wear. Ensure that the key is the correct one for the vehicle.
2. Inspect the key switch for any visible signs of damage or debris that may be interfering with its operation.
3. Check the wiring connected to the key switch for any breaks, frays, or loose connections. Repair or replace any damaged wiring as needed.
4. Clear the trouble code using an OBD2 scanner to see if it returns. If the code comes back, further diagnosis may be needed to pinpoint the exact cause of the issue.

If you are unable to resolve the problem on your own, it is recommended to take your vehicle to a qualified mechanic or dealership for a professional diagnosis and repair.