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HP Designjet 500 and 800 Series Printers - Unknown Paper Format Message

Unknown Paper Format message is displayed on the front panel when trying to load sheet paper or when paper is not installed. Roll paper loads without an error.
Unload media if installed. Check for a media jam around the media sensor. Looking from the rear of the printer, the media sensor is located 4 inches (10 cm) from the left side of the entry platen.
If the media sensor is not blocked it may be faulty and need to be replaced. Contact HP for assistance.