This read-only negative integer holds the specific failure code from the STU's last test, and is only meaningful when stuPrevTestResult indicates failure. It gives support a concrete diagnostic code to look up rather than just knowing 'the restart failed,' narrowing down the actual cause. For example, after seeing stuPrevTestResult report failure on a STU, an engineer reads stuPrevTestFailCode and cross-references the negative code against COM21's fault-code table to identify the root cause.