Cold weather needs more fuel because it doesn't evaporate as quickly is the engine needs. Carbs had chokes to shut off some of the air so the system adds more fuel. Fuel injection just runs a program that will open the injectors longer. But on any system if there is a vacuum leak that air is unmetered so it doesn't know to add additional fuel. Your old O2 sensors were probably still working. When the mixture got real lean they stopped sending voltage or the voltage was real low.
