Well, when I had my cat replaced by the dealership, I had a few problems with smells and gas mileage and then the engine light came on a day after the cat was replaced.
The problem was that I needed a rear o2 sensor for the cat that was replaced under warranty and that solved my problems and the car been running great ever since then.
So maybe you need a new o2 sensor.
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Exterior- stock painted black wheels, clear corners, stainless steel grill, cross and drilled front and rear rotors with evolution brake pads.
Coming soon- 18 inch black mustang bullet rims, GT/GTP rear bumper, tinted windows.