My check engine light came on so I had it diagnosed and the code is po420. the problem said the following,the pcm has determined that the catalyst system efficiency for bank 1 is below threshold for current engine operating conditions. (bank 1 identifies the location of cylinder#1 while bank 2 identifies the cylinders on the opposite bank). I was told it could be the o2 sensor, an exhaust leak or the catalytic converter. Has anyone else had this problem, could it be something else? Please help me! What would it take to fix it? I do not want to pay an insane amount of money for something it may not be.