Runtime Verification (RV) dynamically analyses the sequence of events recorded during system execution, typically stored in traces, and provides a verdict on system behavior. RV has tended to use Boolean, or sometimes quantitative, verdicts to express whether an execution satisfied some specification. However, engineers often want to know the reason for the verdict, which can be found by carrying out \emph{diagnostics}.

In this paper, we develop a diagnostics approach for a time-based fragment of iCFTL, a specification language designed for capturing properties concerning inter-procedural, source code-level behaviour of programs. We begin by developing an instrumentation scheme that builds on iCFTL’s original scheme, enabling the construction of more informative traces. These traces are then used to determine a \emph{point of no return}, which is an event past which a specification can never be satisfied. Our diagnostics approach then highlights a section of the trace in question that leads to the point of no return. We conclude the paper by presenting an evaluation of a prototype tool. Across 21 diverse programs, we observe that our approach is effective, efficient, and induces low time and space overhead.