So as stated in manual l come to complete stop and turn vehicle off then re start the light does notcome back on until l have been driving constantly for 15mins...Why does it do it and will it do damage if l ignore it?

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It can only mean one of two things.
1. Either is needs to be serviced
2. When was the last time the checked to see how much transmission fluid you had in it? You might need to add a quart and make sure that it the correct kind and wgt.

Anything that goes unattended will become damaged.

More money!

Time to get it scanned at a Mitsubishi dealer. Flashing orange transmission light indicates overheating of ATF or shift problems with the vehicle speed sensors or the shift solenoids being of concern. Have it scanned and also see if there is a modified automatic transmission pan bulletin for you model, the Montero Sport had a higher capacity tranny oil pan for an issue with ATF foaming which triggered the orange light .