listen up

So today I heard a radio advertisement from pep boys. The ad read: “For a limited time, come in to Pep boys for a free ‘Check Engine Light’ diagnostic with any paid repair service”.
So lets step back. Checker and Auto zone (Pep boys main competition) do this service for free. The dealership does this for free. Wouldn’t it be wise to also do this for free? With the hope that said customer is there, in your parking lot, talking to a service tech, and would be more than likely to purchase needed items for the repair, or elect pep boys to preform the repair?
While I understand pep boys not wanting to give the farm away, i dont see how charging for such an easy service that will lead to more sales is the best way to go about things.