This is a common fault as the cars get older and usually happens when the trip button is reset to zero. It's a fairly straightforward fix, IIRC Clark's garage gives a pictorial guide on how to do it. There are two gear types and so you'll need to check which one you need, IIRC the 20/21 gear is the most common, something like 99% of 944's have this gear but there is also a 20/22, you need to get the correct gear for your car. You can get the gears off eBay, they are not cheap, this link is for a UK supplier, you may find it cheaper from overseas.
http://www.ebay.co.uk/itm/Porsche-944-1 ... SwFnFWDrIePete