My 1999 Accord Engine Light came on and I went to Autozone for a diagnostic. Got this code.. "P0420 Catalyst System Efficiency Below Threshold".. The dude told me to get the cat convertor replaced. This is the 4th time catalytic conv failed in this car, the last time was about 2.5 years ago. The car has 152K miles on it and is 10 years old. Is this normal ? Going to cost me $600 bucks to get fixed and pass emissions test.