I have a 1.6 automatic golf & it will not select 4th gear. Computer says G38 speed sensor fault. Has anybody got any idea if this is correct & if renewing it will cure it?

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The diagnostics read G38 speed sensor fault with a part number of 00297. A few months ago the car would appear to loose drive in top gear on hills when warm after coming off a motorway for example. Then one day driving home from work it would not select top gear at all. I stopped at a friends repair shop but he just said scrap it. When I left his place the car selected top gear no problem & was OK for a further month or so. Now its lost top gear all together.