Most times in ECU system function testing, the sensor input signals are directly set to a known value in order to drive the corresponding software variable to within a range of an expected value. This works only if the transfer function from the physical signal input to the software variable is well defined such as the measurement on MAP, A/C pressure, etc. Nevertheless, there are times the transfer function is not clearly defined and it is difficult to drive the software variable to an expected value. One example is throttle position sensor (TPS) test in an electronic throttle control (ETC) system, where TPS is not directly driven by the driver accelerator pedal sensor (APS) and it is very difficult to get TPS to an expected range by only changing APS. This paper introduces a method to use feedback in an HIL based ECU testing system to control outputs to an expected range. In this case study, the signal to be controlled is connected back to the HIL system to provide feedback. The error between the target and the actual signal is used in a PID control system to adjust the input signal dynamically and keep the signal to be controlled within the targeted range. Two different feedback methods, namely hardware feedback by physically connecting the signal to be controlled to HIL simulator and software feedback by reading back the software variable to be controlled through ASAP3 protocol between the HIL simulator and ECU instrumentation tool, are evaluated.