O2 is fine (new). The problem is during the period before closed loop. What other inputs control the warmup regulator, or is it completely stand-alone? I'd rather chase down a bad sensor than replace the WU regulator, which I understand is expensive.

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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