This module introduces a structured approach to troubleshooting building systems encountered during commissioning and Retro‑Commissioning projects. Participants will learn how to analyze system symptoms, identify probable root causes, and verify performance using observations, manual testing, trends, and data logging. The course covers troubleshooting concepts across airside, waterside, electrical, plumbing, fire protection, security, and building envelope systems, emphasizing safety, documentation, and coordination with contractors, vendors, and Authorities Having Jurisdiction. Real‑world examples are used to demonstrate effective troubleshooting techniques and performance verification.