The complexity of Lucent Technologies' advanced 5ES® switching system makes the cost of finding, fixing, and retesting a software fault very high. The current 5ESS system contains several million lines of source code, which provide many complicated real-time switching function features. As customer demands for ever-increasing product quality compound the high cost of testing and reworking source code, it is crucial for the 5ESS Switching Development organization to find ways to prevent faults from being introduced into the software in the first place. The 5ESS Switch Coding Fault Prevention Team was assembled to find methods to prevent the most frequent faults from being injected into a product during coding. The Coding Fault Prevention Guidelines, developed by the team for use in various Lucent switching development organizations, lists the most frequent errors made during coding. It also provides coders with information that will help reduce the risk of introducing faults into the software. This paper describes the most common preventable faults and the technical guidelines developed to overcome them. It also explains the metrics used to evaluate the results achieved.