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I keep getting a P0420 code on my 996. It started after replacing the OEM exhaust with a Fabspeed system (cats, mufflers, headers, X-pipe, etc.). Have tried everything. It comes back after awhile after I clear it (sometimes a week later, sometimes a month later). Try putting on an O2 sensor extender. That seems to work for me.
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Mike
'68 Ford Custom 500; '70 Plymouth Cuda; '71 Ford Econoline; '73 Porsche 914 1.7, 1974 Porsche 914 1.8; '99 Porsche Boxster, '03 Porsche Boxster, '03 Porsche C2 Carrera; '07 Porsche Cayman, 2014 Porsche Cayman.
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