A diagnostic method for a container processing machine including a rotating conveyor, and processing units carried by the rotating conveyor and designed to engage at least one container to carry out processing operations is disclosed. The method includes operatively coupling a first set of monitoring sensors to the processing units on-board the processing machine to monitor relevant operating parameters. The method also includes controlling the processing units to execute test operations in lieu of processing operations. The method further includes acquiring measurement data from the first set of monitoring sensors during the test operations. The method further includes processing the acquired measurement data to provide maintenance information on the reliability of at least one of the processing units or the processing machine.