Marking for Operating Tempature Range on Diode's data sheet?
78K2120 is the Newark product code for that diode. 

http://www.newark.com/vishay-general-semiconductor/1n4004-e3/diode/dp/78K2120?Ntt=78K2120

Google FTW. 

That diode's temperature range is -50~+150 degrees celsius. Data sheet: http://www.vishay.com/doc?88503; Farnell Australia (au.farnell.com) part number 1421919.