Electronic Control Unit (ECU) networks should be tested as a system to ensure efficient and accurate results. Often, testing of ECU networks is performed with specially equipped test vehicles. In–vehicle testing is particularly costly, time–consuming, and inefficient with heavy–duty trucks. Also, complete code coverage is typically not achieved with in–vehicle testing. This incomplete testing can lead to numerous problems including an increase in warranty costs and the failure to meet regulatory requirements. A scalable Hardware–in–the–Loop (HiL) system can provide a means to test such large ECU networks in a laboratory environment.