I am trying to print an A4 document. Paper trays 2 and 3 both have A4 paper and the appropriate settings but the printer asks me to Load Letter size paper in Tray 1. My application is also set for A4 size paper.
I can'r even see any way to overrride but I would prefer not to get this message at all.
If the printer displays a prompt asking for a specific paper size (and probably also paper type) to be loaded (perhaps in a specific paper tray), this implies that the incoming print job has requested a paper source where at least one of those attributes does not match the tray configuration as set on the printer.
From the printer, print off a Configuration Report - this should show the current tray setings.
To find out what the incoming job is requesting would require 'capture' (to file) of a typical job, and then analysis of that file.
I'm not sure what analysis tools are available for Mac systems; if you are using a PCL5 or PCL6 (PCL XL) driver, and have access to a Windows system, you can use the PRN File Analyse tool in the PCL Paraphernlia application (available via http://www.pclparaphernalia.eu).