1badmagnum

I have a 1990 es 250,great running car except when ambient air temperature falls below freezing. the car hesitates badly,even when engine is at correct operating temperature. the check engine light has been on for years,the dealer cannot find anything wrong. I am concedering replacing the o2 sensor or the maf sensor. any opinions on which one would most likely cause these problems? last winter I had the same symtoms,but the car runs all summer without any problems whatsoever.