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A lot of technicians replace 02 sensors thinking that they fixed the problem since the vehicle passes smog. What happens a lot of times is that the weak (I say weak because an 02 sensor is really a battery)can ruin the cat because of a rich condition that burns the cat internally and can plug it overtime. It is a good idea on 240's to replace yout 02 sensor from time to time since you can wire in a 1 wire Bosch inexpensively. Don't forget about decarbonization and injection cleaning. One 240 I heard about went to a few shops and still didn't pass smog for CO failure. The several shops installed a new AMM, 02 sensor and Cat. 240's clog with carbon and after this aforementioned 240 was decarbonized and the injectors were cleaned it passed the emissions test CO portion easily.