Caixa automática de Lancer 2008

Greetings to all ... I have a Mitsubishi Lancer 2008 that presents problems with the A/T gearbox
was in a workshop for chapisteria work now that it came out perfect from one day to another now the reverse only works and does not go forward I have understood that I use the internal filter of the A/t if someone can guide me more if you have to disassemble it to change that filter I appreciate the help ...