why does the gearbox light stay on my laguana?

We have an 02 reg renault laguana 1.8 automatic, quite often when we insert the key and press the ignitionbuton the gearbox light stays on therefore the engine is immobolised our local garage can not plug its fault finder in because the problem did ot occur whilst it was in the garage. We have tried changing the battery in the key also the vehicle was given a full serice so fluid levels are correct.