I'm going to have to argue that one. My 1999 has had the P0420 Cat Below Threshold code since 2001, that's 7 years with the code and has not suffered a loss in mileage. In fact, the other week I got 34 MPG.
Yeah, me too. Losing thePRIMARY (front) O2 sensor causes it to go into open-loop. The rear O2 sensor doesn't close the control loop at all. It's just monitoring the cat.
Considering the secondary O2 sensor is located downstream of most of the catalyst, it's not evenmeasuring the proper thing to close the fuel-control loop.
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'05 Saab 9²x aero (WRX)
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RE: Catalyst Efficiency Below Threshold--
