Does it run better with the new sensor installed? I give my cars the subjective 'sniff' test. If the exhaust smells like clean hot air, they have always passed emissions.

Regarding duty cycle, I thought I read that earlier models are something like 30/70. I haven't looked at mine in a while, but I'm pretty sure its not at 50% and it passes the state test very easily.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma