So it's showing to little? I really don't know what the problem could be,
but check the wire going from the temperatur sender in the engine block.
Disconnect og connect it several times (should be an easy pull-off
connector) and check if the temperature gauge reacts the same way every
time. It should point at the coldest area when it's not connected. Follow
the wire as long as possible (until it goes inside the wire-harness), make
sure the insulation is not cracked. This is one of the easy things you can
do without dealing with the complicated wiring inside the dashboard etc.