Aspects disclosed herein relate to periodic non-intrusive diagnosis of lockstep systems. An exemplary method includes comparing execution of a program on a first processing system of the plurality of processing systems and execution of the program on a second processing system of the plurality of processing systems using a first comparator circuit, comparing the execution of the program on the first processing system and the execution of the program on the second processing system using a second comparator circuit, and running a diagnosis program on the second comparator circuit while the comparing using the first comparator circuit is ongoing.