All I can do is speak about my experience's but when I started having issues I noticed a big decrease in mileage. I was going through gas much quicker as I had to give it more gas to move. I had reset my engine light and it came back on immediately. After gutting it, my mileage is back to normal, my car is running as I feel it should and no more lights. I assume as it clogged I was getting less and less mileage up until it finally clogged all the way. I have read online about people saying there are no adverse affects to the vehicle running with a gutted cat and those that say its not a good idea.