check engine light

The check engine light came on in my 03 silverado. I ran the code and it gave me code P0420, Catalyst System Efficiency Below Threshold (Bank 1), but I kept expecting to see a loss of power or loss in fuel mileage, but neither of these has happened. Also I noticed that I can clear it and it wont come back until I brake hard or suddenly. Why would hitting the brakes cause this code to come up?

The check engine light came on in my 03 silverado. I ran the code and it gave me code P0420, Catalyst System Efficiency Below Threshold (Bank 1), but I kept expecting to see a loss of power or loss in fuel mileage, but neither of these has happened. Also I noticed that I can clear it and it wont come back until I brake hard or suddenly. Why would hitting the brakes cause this code to come up?

I would look for a vacuum leak in your brake system, wither booster or hoses.