This article discusses methods that allow controlling, diagnosing and correcting errors that occur in a computer system (CS) operating in a modular number system (MNS). A feature of using the MNS is the possibility, in some cases, of correcting errors even if there is only one control base, using the concept of an alternative set of numbers. The article deals with the control, diagnostics and error correction in the dynamics of the data processing process. Currently, to correct errors in the dynamics of the computational process, the CS uses the projection method and its modifications. The projection method requires the calculation of all projections i A of the distorted number A , which leads to a large number of additional operations for each correction of an individual result. The article discusses a method for correcting errors in the CS, based on the use of a conditional alternative set of numbers in the MNS. Two methods for determining an alternative set ( ) W A are considered. The disadvantage of the first method is the large hardware and time costs for determining an alternative set of numbers. As a working method, the second method is proposed, in which the initial number A is first reduced to the form ( ) 1