Code 420
 

2014 XC60 3.0 mileage 162000
Got a code for “miss fire cylinder #6” Replaced all plugs and coils. A week later I got a code 420. How do I rule out intake air leak? No antifreeze missing or signs of it in oil.

To find an intake air leak, take a can of carb cleaner (with the spray tube on) and spray around various components and the injector seals - looking to see what gets sucked in and whether it bumps the idle). If that fails go to a local shop who can do a smoke test for you. If that fails you may need to do a compression/leak down test to see if there's ring or valve damage. Did you inspect the wire harness connectors going to the coils? I suppose it could also be an injector (kind of rare but can happen) so you also may try swapping between 5 and 6 to see if the fault code moves to #5.