300TD - Pre Heat Light on Dash

[toby tucker]

Turn ignition on , pre heat light on dash comes on- goes out after approx 5 secs; Start car - all's Fine. I then drive off and pre heat light comes back on again and stays on for 2-3 miles or so.

Can anybody give me an idea what the prob is and how I can fix it ?

The car is a '99 with 80k on the clock and drives perfectly.

thx Toby