Error might be encountered when a zone defined in one of the monitors doesn’t exist. For instance, say a surface monitor is defined with an interior surface. Later on the interior surface is changed to wall and iterating the case might lead to Error: domain-thread-integrals: invalid thread Error Object: 601 As seen from the error message, it seems that the thread with id “601” does not exist and a monitor definition is trying to use it. Before iterating please check the names of the thread with id 601 which can be done through the TUI by issuing the following command: (thread-name (get-thread 601)) If the zone with the id is indeed missing, then the problem is definitely because of the monitors. This error occurs when a monitor points to an old thread (zone). Please try the following to resolve the problem: 1) Delete all the monitors; change the interior to wall, save case file, read into a new session of FLUENT and redefine the monitors. 2) If this doesn’t remove the problem do the following. – Delete all the monitors – Change the interior to wall – Use the text command file/write-settings to write out boundary conditions and solver settings. – Read the case into TGrid and save it as a .msh file – Read the .msh file into a new fluent session. Read back the boundary condition file via file/read settings – Redefine the monitors.