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9/10 this is down to the cat the only way to know for sure is to view the live data on diagnostic tool and analyse rear lambda operation .but more likely tobe the catalytic converter at fault
Anywhere with a scanner showing voltage for lanmbda bank 1 sensor 1 and bank 1 sensor 2 .bank 1 sensor 1 voltage shold fluctuate batween o and around 1 volts and the bank 1 sensor 2 should be static around 0.7 volts . If sensor two is moving then catalytic converter will need replacement
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