when I turn off my ignition the fan continues to run.It hasn't done that before on 1999 Nissan Altima

I recently had a blow head gaskit replaced and a thermostat, coud this be lonked to the problem? do I need to take it in to be checked?

Asked by for the 1999 Nissan Altima
The fan switch , relay or control to it could be faulty. Should be tested first to confirm which it is.