The OBD2 code U1163 on a 2003 Buick Lesabre typically indicates a communication error between the Electronic Brake Control Module (EBCM) and the Engine Control Module (ECM). This can be caused by issues such as a faulty wiring connection between the modules, a defective EBCM or ECM, or a damaged communication circuit.

Here are some steps you can take to troubleshoot this issue:
1. Start by checking the wiring connections between the EBCM and ECM. Look for any loose, damaged, or corroded connections and repair as needed.
2. Inspect the communication circuit between the modules for any signs of damage or short circuits. Repair any issues found.
3. If the wiring and communication circuit appear to be intact, the next step would be to have the EBCM and ECM tested by a professional to determine if either component is faulty.
4. Reset the fault codes and see if the U1163 code reappears. If it does, further diagnostics may be needed.

Remember, if you are not comfortable performing these steps yourself, it is always recommended to consult a professional mechanic for assistance.