I have a 99 Durango, 5.9, 4x4, 197000 mi, everything is stock. I'm getting codes P0420 Catalyst Efficiency Below Threshold (Bank 1) and P0308 Cyl 8 Misfire. I have an exhaust leak that is coming from the collector. I'm going to start with checking plugs and wires on #8. Can the 420 code be caused by or linked to the misfire or exhaust leak? The D does blow dark smoke when it first starts up and I also burn alot of oil - close to a quart every 1000 miles. Thanks for any help!