—When testing a time-critical system, some scenarios can be hard to run when acting only on the input sequence. The proper execution of a given scenario might require for instance a minimal execution time for a given piece of software. Execution times are notoriously difficult to control because they depend not only on the inputs, but also on the state of the micro-architecture. In this paper, we propose a method, based on runtime enforcement, which forces a system to run such a scenario. We also describe an implementation in the context of a RTOS for embedded control systems. Our method starts with a parametric formal model of the system where the parameters are delays that can be added to simulate longer execution times. The domain of acceptable parameter values to run the target scenario is computed offline. Online, a framework plugged in the RTOS observes the execution of the system and injects delays when needed.