12 Another sensor that and EFI system needs in order to properly function is an O2. The O2 sensor tells the ECU what the fuel mixture is like at any given second so adjustments can be made. In the case of the EZ kit, adjustments are made automatically by the ECU—just like the new car in your driveway. The O2 sensor should be mounted. Installing the sensor just after the collector gives it a good reading from each cylinder.