the yellow engine warning light has come on ,anyone no how to rfix it,thanks

I'd say give the battery a charge first see how that goes. If not maybe a dodgy connection somewhere?

Reg / Rectifier connection. When the dash starts acting up you do not have enough charge in the battery to start the engine.
Take apart and check that the terminals have not corroded or blackened. If they have at least clean them up, check in particular the earth terminal.
If you don't you are walking.!

Cheers Steve

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the engine is running and starts up fine, the light does not go off,when you rev up