That's much easier:It's almost right, although I don't want it looking in the entire column D:D, only D2, to see if "knott", if not use vlookup.
=[COLOR=#ff0000]IF(D2="knott","knott"[/COLOR],[COLOR=#333333]VLOOKUP(AJ2,'TPPE Mapping'!A:D,4,FALSE))[/COLOR]
Ah, but it is!Not true, because it is filling in knott if it finds knott, but it isn't doing the vlookup if knott isn't there