So far with used and abused Expeditions or just Triton v8 cars in general are the Catalytic Converter going bad. This is due to Negligence from the previous of the vehicle and not keeping it in top shape, especially when there's a misfire code and it's being ignored. Those prolonged periods of misfires can cause the cats to become damaged over time. So just one of the MAJOR things toi look out for are codes, and namely a System Efficiency Below Threshold code. Both me and my brother ran into this issue with both previous owners of our own truck, and we ended up having to replace our cats. It can be a pretty expensive job if the entire system is shot.