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Is the problem the oxygen sensors or Catalytic converter? on 2001 Infiniti I30

2001 Infinite I30. I had O2 Sensor Bank 1, Sensor 2 replaced in May of 2010 by a new mechanic. They recommended that, in these cars, all 4 sensors work together so ALL four should be replaced but I couldn't afford all, so I replace one and the "check engine" light went off. Light back on now. I took it to my old mechanic. The diag. code is coming up "middle catalytic converter". He says this should be replaced. I told my newer mechanic this. He says the failing O2 sensors are causing the CC to malfunction, hence the code and that I should replace all remaining sensors instead. Old mechanic says he detects no malfunctioning with remaining O2 sensors. Who's right? Help!

Asked by for the 2001 Infiniti I30
your old mechanic. you can test the o2 with a scanner and a scope for operation. you do not need to replace all when one fails, just the failed part.
your shop can do a graph of the cat to see if it is bad or not. cats are common for failure from either age, 100k or more, or from misffires from a bad plug or coil over an extended period of time.

Roy
My old mechanic comes through once more....Thanks for your help Mastertech!