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2000mc · 01-27-2015, 01:52 PM

I always suspect aftermarket cats first. Second guess would be a problem causing one cylinder to run lean, pumping out nox. So then to make the O2 sensor happy, the rest of the cyls run a bit rich, pumping out some co and hc.

01-27-2015, 01:52 PM	#3 (permalink)
2000mc herp derp Apprentice Join Date: Oct 2008 Location: Lawrence, KS Posts: 1,049 Saturn-sold - '99 saturn sc1 Team Saturn 90 day: 28.28 mpg (US) Yukon - '03 GMC Yukon Denali 90 day: 13.74 mpg (US) Thanks: 43 Thanked 331 Times in 233 Posts	I always suspect aftermarket cats first. Second guess would be a problem causing one cylinder to run lean, pumping out nox. So then to make the O2 sensor happy, the rest of the cyls run a bit rich, pumping out some co and hc.