A fuel injector control modification module is configured to modify primary fuel injector control signals provided by an engine control unit of an internal combustion engine. The module is particularly adapted to modifying fuel injector control signals based upon detected variations in fuel provided to the engine. Information indicative of fuel type is obtained by the module from an onboard diagnostic port associated with the engine control unit. The information may comprise fuel trim information which is generated, at least in part, from exhaust gas oxygen content information provided to the engine control unit. The module may modify or completely replace the primary fuel injector control signals at one or more times with secondary signal, such as to accommodate changes in fuel type. The module is particularly suited to addition to an existing engine to adapt the engine to run upon alternative fuels not contemplated by the original engine manufacturer.