As you build a model, the data and tools you add and connect are automatically validated or checked to make sure that the data exists, is valid for use with a tool, and that all tool parameter settings are correctly specified. You can manually validate your model by clicking ModelBuilder > Run > Validate. This is typically only necessary after you have created a model and the data or tools inside the model have been modified, moved, renamed, or deleted. You will also need to validate your model if all processes in the model have been run and you wish to run them again.
Validating will return all processes to their ready to run state. If you validate a model and a process changes to not ready to run (elements are gray), it means that one or more tool parameters or input variables are invalid.
Validate data variables
Data variables reference data paths and contain descriptive information about the data. When you validate a model, each data variable is checked to ensure the data exists and can be used by the tools the variable is connected to. If any data variable references a data path which no longer exists, the variable becomes invalid and the color changes to gray.
If anything about a dataset changes (for example, you add a new attribute field to a dataset outside ModelBuilder), you may need to validate the model to see those changes. Similarly, if a dataset used as an input in a model is deleted or renamed, when you validate the model, it will invalidate the variable and change its color to gray. To repair this invalid data variable, double-click the variable and select a new layer or browse to a new dataset.
Validate a tool
Each parameter of a geoprocessing tool automatically validates the data or value that is specified for that parameter. If the data or value is invalid, an error message is displayed. If a tool parameter is invalid, the geoprocessing tool and any downstream processes (tools which are dependent on or process the output of that tool) are marked as not ready to run and their color changes to gray. To repair a tool that is not ready to run, double-click the tool to open it, find the parameter that has an error, and specify a different dataset or value. You can also right-click any geoprocessing tool in a model and select Messages to view any error messages.