Error messages
mental ray for Maya checks for errors in a scene and recognizes various operating system errors. If an error is detected, a message appears in Maya's script editor. If a critical error is found, mental ray for Maya aborts the current process.
The message verbosity can be changed by selecting a different Verbosity Level option in the Render > Render Current Frame and Render > Batch Render > 
. When using the interactive renderer mental ray for Maya prints messages to the Output Window (Windows) or the Console (Linux). When batch rendering with mental ray for Maya, a log file is created. If severe errors are encountered during database access of mental ray, the final rendering doesn’t get started.
Diagnostics
In the Render Settings window, under the Quality tab, Sample Options section, select Diagnose samples to render diagnostic passes to an exr file. You can then view these passes via File > Load Render Pass > Diagnose samples in the Render View window. See Sample Options for more information.