Darren Cotton
Got to ask the same question a Mark....
If the rear sensor is measuring a high O2 how does that make it efficient?
I don't know why after all these years techs are changing O2 sensors for this fault code, P0420 is what it is, "cat efficiency" simple test scope front and rear sensors engine warm, fast idle....
and all will come clear....
Darren
Message Thread 
Citroen C3 1.0 2003 - CAT aging fault, code P0420 # - Alf Cox June 19, 2011, 8:11 pm
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