Speedo is keep flinking about?

I have a renault megane convertible 1.6E 1999 model i recently notice the speedo is keeping flicking and i get the electrical sign on the dashboard coming on, also i manage to change the sensor and the problem carried on , i manage to get hold of one of those devices that read the data and came with a error 20G something to do with the gear box for some reason is thinkin is auto when is manual any HELP........