Engine Mechanical problem
2002 Mazda 626 4 cyl Front Wheel Drive Automatic 97868 miles
Car: 2002 Mazda 626 LX 4cyl 2.0L
The check engine light with code P0421 goes on when the car run city miles, and goes off when running highway miles or higher MPH in city. I sent my car to an auto shop, and they said that I need to replace the CAT, which runs about $1000, because of some problems with exhaust system. But the car passed the emission test over 1 month ago, and oil change is needed now. If it's not the problem of the CAT, what are the other possibilities?