Engine management light on 2001 Vauxhall Astra 1.7 DTI (DIESEL) LS ECO comes on only during cold start?

Hello all, I have 2001 X Reg Vauxhall Astra 1.7 DTi LS ECO. I have an issue with Engine management light coming on only during cold start. It goes off after a few minutes of driving/ idling & would come back on again only if you leave the car OFF for a few hours. In winters, the light stays on for longer but eventually does go off & never comes back on again once the engine warms up.
During warm start ups (where the vehicle has not been off for more than an hour or 2), it goes off with other lamps as soon as you crank the engine & does not come back again.
The vauxhall manual says there is nothing to worry about if the light does not stay on permanently or flash intermittently. I must add that the car starts & runs absolutely fine and there is no impact of this engine management light on day to day running of car. But then why does this light come on?
I had it checked once by the garage & by the time I reached there, the light went off and there were no fault codes. Thank in advance

Answers:
Chaces are that E.C.U. is picking up the fact that the engine is running richer because cold and after a few minutes is within perameters hence light going out.Could be faulty temp sender sending wrong reading to ecu when cold or core programme perameters may need reprogrammed,to do this you need to go to vauxhall or independant garage with a TECH 2 diagnostic equipment with relevant software upgrades
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