Vauxhall sat nav?

I have a 56 plate vauxhall astra with built in sat nav and i can't seem to just put a post code in to find an address. It let's me put the first part of the post code in then I have to find the area then the street name and it's a lot of messing about. Do you know if I have settings to change so that I can just put the full post code in. Thanks in advance for any help you can give me with this.