The O2 sensor EFIE is perhaps the most essential part of supplemental hydrogen effectiveness. If you do NOT address the pre-catalytic converter oxygen sensor(s) when installing HHO fuel cell or hydrogen generator systems, you will not see any increase in fuel mileage but to the contrary possibly see less mileage gain because of the increase oxygen content produced by hydrogen induction. Computerized engines of the OBDII vintage will see this induction and try to offset it by dumping more fuel into the injectors. The Digital O2 sensor efie will offset this problem electronically and effectively when properly installed and configured. .