You are always better to fit OEM parts . There are a lot of non genuine Ford parts from abroad that are not up to standard and can cause problems after fitting . I have had cars where such parts have been fitted and owners thought it cant be that now as is " new " part fitted . Taking it off and fitting genuine part cured the fault . If the parts fitted by your mechanic are faulty he should return to them to AutoZone under warranty for refund and then reimburse you the cost