A diagnostic system for monitoring a mechanical trip mechanism of a circuit interrupter provides advanced diagnostics. The trip mechanism comprises a D-shaft that is actuated by the trip unit, an operating mechanism that is actuated by the D-shaft, and a pole shaft that is actuated by the operating mechanism to physically separate the separable contacts. During every opening operation, the diagnostic system determines how much time elapses between the D-shaft beginning to move to its open position and the pole shaft reaching its open position, in order to detect degradation of the trip mechanism as early as possible. This ensures that suboptimal performance is detected even if the trip mechanism is meeting the minimum requirements for acceptable performance. A user is notified once the performance has degraded to a level that is considered concerning before the trip mechanism fails altogether, saving significant time and resources and preventing more widespread damage.