HVAC certified prior to close and now it is not working...any recourse?

We had the previous owners have the A/C and HVAC certified by a licensed technician prior to closing on our home. We are moving in and finding out that they are not working properly. We are contacting the previous owners to find out if they knew about the issue just to see if they have an easy way to fix it, but we are not expecting a remedy or even a response. We did contact the HVAC tech and we are wondering since they certified it if they have to fix it. If anybody has had the same issue please let me know, and what were your results. Thanks