PO420 check engine light error - catalytic converter already replaced twice

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Hi, I have a 2001 with close to 250,000 miles on it. Throughout it's life the check engine light has periodically come on and gone back off (error codes were never read). Recently, I acquired the car and when the light came on had the code read (PO420). I noticed a smell around this same time of oil or burning rubber coming in through the vents (mostly when the heat is cranked up). I don't remember a smell prior to the light (but I was driving it more sparingly then in better temperatures). I took it to a local garage and he said to replace the catalytic converter - I asked about the O2 sensors and he said it would not be them. I replaced the converter and within a two months the light was back on. He thought it might have been a faulty converter so he put a new one in. Two months later my light is back on. The mechanic said the next step would be getting the computer replaced at a dealership. How much does this cost and is this the most likely cause of the trouble?