Horrible customer service

I took my phone for repairs and they told me that the warranty on the part is 3 months . Before the 3 months was over the phone gave me the same problem so I took it back. They told me that they readjusted the part and they gave me the phone back. After 2 months the phone gave the same problem so they readjusted again and gave me my phone back. After another 2 months again I took the phone back with the same problem .they are now telling me that they must order a new part at my expense because it is out of warranty .First of all they should have detected that the part was faulty while the part was under warranty. I should not be expected to pay for their mistake.it is the technician job to detect a faulty part.HORRIBLE customer service .