2000 Vitz check engine light

[BomB191]

As subject I have a year 2000 Vitz 1.0L auto

problem is lately every time the car goes into "over drive" or top gear the check engine light comes on Normaly this is around 60-80Kmph other times it will come on when im doing 90Km ish and turn "over drive" off to pass someone in a passing lane It also comes on then if it hasn't already.

Oils been changed (3 weeks ago)
plugs changed
air filter clear
I'm flushing and cleaning the radiator tomorrow (but doubt that's the cause of my issue)

Heres the catch though she has done just shy of 600,000Kms and have been debating on changing the AT fluid but dont see the point if it isnt the cause of the check light issue..

Has anyone else had this problem and a way to fix it?

ps I went through 20 od pages before posting this just in case this is a repost sorry.