OEM or knock-off cat installed? Is there any kind of exhaust leak pre-cat? Was it done roughly around the time weather got cooler/gas switched to winter blend?
If you have any exhaust/tire shops you have a good relationship or experience with, I would just take it there and have them look it over. Yes, an exhaust leak could definitely cause a drop in mileage since the O2 sensors wouldn't be seeing what they'd expect to.I have suspected an exhaust leak since the installation. I keep getting "whifs" inside the cabin occasionally. This is why I took it back to the dealer. I susspect that all they did was inspect the installation and not anything else. Their explanation for the exhaust smell was that the new converter comes with a protective coating that has to burn off. My problem is that I do not have the money to pay them for "further inspections or remedies" as they put it. This was an insurance claim that has now closed with my insurance company. And since this dealer is not on their preferred list, they will not help me further.
But how many shops are willing to touch a "low MPG" claim? I think he needs something more definitive. Perhaps a exhaust leak sound.Does that dealer not insure their work for 12 months/12,000 miles as GM recommends? It doesn't matter if it's an insurance claim or not - the work should be covered.