I have a 1996 volvo S70 the check engine lights keeps coming on every time I get it check they state that it is the gas cap I have replaced the cap twice I looked up the code and it stated evaporative emission control system purge control valve circuit shorted could you tell me more on that

That code indicates a large evap system leak. To properly diagnose the actual failed component, special tools and a patient tech will need to spend some time tracking it down.