Check the voltage of the O2 sensor. Should be between .1 and 1.0 volt when the engine is hot. If it hasn't been replaced lately, it makes sense to invest in a new one and eliminate that as a possible cause.

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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