1. A method to detect a fault in a CPU of an industrial controller during on-line safety control of real world objects the method comprising: compiling an application program into assembler instructions, which application program was previously defined in a high level language intended for safety control, compiling a test application into assembler instructions where the assembler instructions is a subset of the total number of assembler instructions available for the CPU, which test application was previously defined in said high level language intended for safety control and the test application covers at least all language constructs used in the application program, downloading the application program and the test application to a central unit of an industrial controller, executing repeatedly the test application in the industrial controller, comparing repeatedly by means of a test module a result from the test application with the pre-defined result in the test module, detecting a fault in the CPU as the result from the test application does not equal the pre-defined result stored in the test module and the unexpected result of the test application is due to the execution of an assembler instruction of the test application, aborting the execution of the application program wherein the application program is prohibited from executing the assembler instruction which otherwise would cause the application program to fail.