Ultra wideband (UWB) technology attracts attention from automotive manufacturers as a potential way to construct an intra-vehicle wireless sensor network, which is also called networked control system (NCS). Given that modern vehicles incorporate tens of sensors to provide vital sensor information such as temperature, air quality, tire pressure, distances to nearby objects, etc., for the electronic control units (ECUs), the NCS inevitably operates in a multi-user interference (MUI) environment. This paper investigates the effect of MUI on the performance of intra-vehicle UWB NCS based on the recently proposed intravehicle NCS channel model. Through Monte Carlo simulations, we firstly study the multi user system performance in two different intra-vehicle NCS scenarios, and find that due to the different channel characteristics the multi user performance of the chassis NCS is more sensitive to the number of the total asynchronous users. Furthermore, we compare the multi user performance of the intra-vehicle NCS with that of indoor or outdoor environments.