What could be aftermarket exhaust is that O2 sensors are reading values, that is causing ECU is trying to enhance A/F ratio and with more too much fuel or too low could cause this.
Dad had same issue 10 years ago on VQ20ER engine. In 25 000kms were catalytic converter burned also with headers and gaskets
If you come to Czech, let me know if you are interested to make CZ/DE 370Z meeting
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2005-6 Nissan Micra 1,3i CVT
72PS Red MY 98 -->
2006-8 Nissan Almera 1,5i N16
96PS Red -->
2007-8 Skoda Fabia Combi 1,2HTP 3 cylinder
60PS Green -->
2008-11 AUDI TT 2,0TFSI FWD S-tronic
200PS Red MY06 -->
2011 Nissan 370Z manual
White Pearl 