Poor mileage after starting battery disconnected
 

I had my starting battery out of my 2007 hybrid. After I put it back in the gas mileage as shown on the dashboard indicator was about 12L/100km. I did take the car for a 1/2 hour drive on the highway but that did nothing to get better mileage. Took it to the local dealer and they said that it would need a $500+ tuneup and valve adjustment. I said "impossible" all I did was to disconnect the starting battery. What they were suggesting was not reasonable considering that 10 minutes before I took the battery out the mileage was perfect. I'm thinking that some computer engine control codes were lost. How would I get back the previous 5.5L/100km status?