Ahh yes the this code again lol. I've got this code before. Catalytic Efficiency Running Below Threshold. Basically the Quest has 2 Catalytic Converters. Bank 1 and bank 2 tells which Cat is having the problem. Yours happens to be bank 1 which I do believe is the left side. Basically the Catalytic Converter is suppose to be running at 100% efficiency. If it starts running at 75% or below in efficiency it will trigger the code you got. It means the Cat is not running as efficient as it should be. In other words the inside of the Cat is burned out. This is a VERY VERY serious issue and I'd advise you get it fixed right away. The reason for this is since the inside of the Catalytic converter is burned out, those pieces inside break up inside and have no where to go but get sucked back up in the engine and will destroy it over a period of time. The code you got is either P0420 or P0430. You need to change that Bank 1 Cat right away.