Is it save to drive when my 1997 volvo 960 had po440 ode problem

Hello, this is generally due to a small leak from the fuel tank vapor system. It doesn't mean there's a fuel leak, but a small fuel vapor leak. Double check that your gas cap is installed correctly and the seal is OK. You can see more info here: https://repairpal.com/OBD-II-Code-P0440