Hello everyone I am brand new to the forum. I have a 2001 Chevy Cavalier 2.2 that starts up a little slower than usual. After driving around a couple of blocks the car begins to buck really hard and then stalls out. My mechanic has changed the batter, spark plugs and wires, crank sensor, air sensor, temperature sensor, both coils, control moduler and both coils, and the fuel line. When it is cold the car bucks the most and then I hear a pop and the car stalls out. The emissions light is on and the code said Catalyst System
Efficiency Below Threshold (Bank 1). My mechanic said the converter is not plugged and is fine. Please help!!