Bush Mechanic service at VW Midrand

I was so disappointed with the service I received from VW Midrand . My engine light was on so I booked in for diagnosis it was on Thursday . The first call was about plugs then I authorised for them to be changed . Then they called me again saying that they cannot find the fault . To my surprise I asked them didn't the diagnosis show the faults they said it didn't . So I am asking my myself why should I pay for it . They then started doing trial and error on the vehicle they said maybe the injectors are blocked really experts telling me that maybe they were not even certain . I agreed for them to clean the injectors after cleaning the injector they tested the vehicle the light was still on then they said that was not the fault they then started to test the sensors . All these trial and errors which the so called VW professional mechanics are doing I must pay for it . Is it fair because if I wanted a guessing game I would have taken my vehicle to the bush mechanics who does not have equipment to test the faults . My vehicle spent days at VW . I am so disappointed with the service .