Vauxhall cavalier, K reg, automatic gearbox warning light?

on cold start the gearbox warning light comes on after a short time. however, after a full warm up the light stays off and the geabox works normally

If I start the car and drive off in 1, 2 and 3 gears when cold, no problem untill I engae D. after a short time the light comes on.

Any advice?