Engines modified to increase economy have altered air/fuel ratio values. The stock engine control module will make incorrect adjustments based on these air/fuel ratio values. The Hydrogen Fuel Controller (HHO) is a solution for optimum fuel control on modified engines. To account for the modified air/fuel ratio values the HHO controls the sensor signals to the stock engine control module. The HHO corrects these signals by comparing the input signals to a table, then sending the altered signals to the stock engine controller. Tables can be changed allowing the HHO to be customized in the field to specific applications. When changes are made, this changes the operating parameters of the engine. The HHO uses the MAF/MAP signal to enable the engine control module to operate with the modified operating conditions of the engine. Replacing the narrowband oxygen sensor in the exhaust with a wideband oxygen sensor allows superior control over the air/fuel ratio. The throttle response signals change with a modified engine. The HHO alters the throttle signals so that the stock engine control module can control the engine. The HHO can control a 25-amp fuel cell if additional HHO is required to meet the fuel demands of a modified engine with a stock deadhead fuel system. The HHO2 has a software monitor display showing all inputs and outputs of the HHO, speed, fuel pressure, RPM, throttle position, MAF and air/fuel ratio. This gives an overall view of how engine performance is affected when making tuning adjustments. This data can be recorded for later analysis with the HHO data viewer or Microsoft Excel. Our HHO software offers a strong set of capabilities and data logging viewing options. Data files use the file extension .flg which is associated with HHO software. The HHO has a RS-232 serial port and CAN 2.0b. The CAN 2.0b communicates with other electronic control modules and RS-232 is used to program the HHO with a computer. The HHO has an optional harness containing the CAN 2.0b connectors. Programmable inputs and outputs allow the HHO to be customized to just about any application in the field. Different setting can be stored in the computer and quickly uploaded into the HHO.