2016 XC70. Check engine light is on. Error code shows 2421. Replaced the gas cap, but still have light on. Mechanic wants to do a smoke test. Any thoughts? Thanks.

 

That would be a reasonable step after a visual check to make sure any visible vent lines are still connected. That code will not affect the way the car runs - you will just be polluting.

That code is an evaporative system code - the system thinks gas fumes are leaking. What brand of gas cap did you replace the factory one with? I had to stop using aftermarket ones - they were not holding the correct pressure. If you are curious to actually test the cap - if your state does emissions tests the state machines usually have a cap tester.

 

Genuine Volvo gas cap.

 

that code points to the ECU detecting that the purge valve is stuck open. That could be caused by a wonky valve or by a vaccuum line/hose issue or radiator cap. The purge valve lines run from up by the radiator, all the way back to the charcoal cannister by the gas tank, so the smoke test is a practical way of finding any issues with the hoses if there's nothing obvious visually.