TestWeaver (testing without test scripts) is a tool that autonomously searches for weak points and bugs in control software and calibration data. Users have to supply a simulation model (implemented e.g. using MATLAB®/Simulink, Silver or HiL) and to specify computable quality indicators. TestWeaver constructs automatically driving scenarios that minimize these indicators. This helps to find bugs early and with much less effort than otherwise possible. A typical ECU (software + calibration data) is checked within 24 hours on a standard PC. The automatic test case generation of TestWeaver can run with MiL, SiL, or HiL setups and allows to achieve a much higher test coverage with less effort than otherwise possible. TestWeaver is in use for software development at Mercedes-Benz, AMG, Bosch, ZF, SAIC and others.