Replacement of electrovalves AL4 transmission fault code 6

Guide to the replacement of electrovalves in the AL4 gearbox that cause transmission fault code 6 (and often with a bang) on a Citroen C3. This was a result of intermittent warning lamps on dashboard, consisting of the snowflake and the S flashing together. Occasionally the gearbox would jolt sharply and slam into 3rd gear.