how can i fix my ren 19 auto gearbox?

hi guys hope you can help ive got a renault 19 it has a 4 speed auto box,but when you pull away it over revs and wont change into second gear, you can get it to change by putting it in sports and flooring it but if you drive it normal it just rides and revs any ideas? could it be a sensor or is it just kaput?