A method for monitors the functions and increases the operational reliability of a complex safety-relevant control system. Examples of such control systems include motor vehicle control systems such as a brake system (ABS, TCS, ESP, EHB, EMB), a steering aid (“steering-by-wire”), etc. The method also detects and evaluates system errors. The method detects a system error and evaluates as a group error, brings about a complete system degradation or a partial system degradation corresponding to the group error, e.g. limitation of the system functions and the system availability, localizes the system error and the error source by tests, logical combining of the test results, plausibility considerations, etc. and gradually cancels the restrictions of the system functions and enhances the system availability in dependence on the result of the individual steps for bounding or localizing the system error and the error source.