Customers at Tesco in Branksome, Poole, complained of engine problems after filling up there in September 2014.
The defence claimed the half-hour window during which the contaminated the fuel was allegedly contaminated was not long enough for the contaminant to be pumped in.
Tesco's petrol station in Branksome was forced to shut for nearly two months while investigations were carried out.
At the time, customers complained of damage running into thousands of pounds being done to their vehicles after filling up with diesel.
Read more: http://www.bbc.com/news/uk-england-dorset-35484180