Welcome to the wonderful world of Land Rovers.
Ever wondered why so many members here recommend either Nanocom, Hawkeye or Lynx?
The diesel Discovery didn't become compliant with OBDII protocols until the release of the Discovery 3 in mid-2004. Up until the changeover, Land Rover used their own diagnostic protocols. Petrol Discos were compliant a lot earlier.
Only the devices above are known to give real and truthful indications for DIY use. The iCarSoft unit will probably work for very basic items but not much else.
When the manufacturer states that it works on a 1999-2004 Discovery they never tell you that they mean the petrol version and not the diesel Disco.