In this paper, the fault isolation and fault magnitude estimate methods are proposed. In the original fault isolation methods, contribution plots are popular. However, it is not accurate. In the original fault estimation methods, the authors assume that the fault magnitude is far greater than the normal measurement. However, the assumption is too strong. To avoid the above two problems, in this paper, the fault is isolated by determining the fault direction and the fault magnitude is estimated using the fault reconstruction. The proposed methods are used to Monte Carlo simulation and the electrical smelting magnesium furnace. From the simulation results, we can see that the proposed methods can solve the problems mentioned above effectively. (C) 2011 Elsevier Ltd. All rights reserved.