Poor after sales service

On the 16 January 2025 I bought Mercedes benz engine from tpc in jet park and after fitting the engine on my bus it did not run and went back to tpc to sales person Mazut to request an exchange of the engine with no success,tpc keep on insisting that the engine is running even though I submitted proof to them that the engine sies infact have issue,this is a frustrating treatment I receive from gauteng truck parts centre as this is a business asset which generate income, I have tried to report the matter on all levels at tpc gauteng with no success and would like to be assisted in this regard