1996 sharan gl 2.0 petrol, holding back and thirsty 00515 fault code?? also battery draining over night??

hi everyone hope you had a good christmas, i have a 1996 vw sharan 2.0 petrol manual, its running too rich for my liking only just getting 15 mpg around town and just managing 24 mpg on a run, also it holds back sometimes having to change down a gear to get it too pick up speed, i have a vw fault code reader and it comes up with only one fault code 00515 it clears but comes back everytime i restart it?

overnight the battery drains too??

we had a 2.3 petrol galaxy before and easily averaged 34 mpg combined driving, so know this is not right