A system comprising: an engine comprising at least one cylinder, at least one fuel injector, an intake manifold and an exhaust manifold, wherein the at least one fuel injector is configured to inject a mass of fuel to the cylinder, subject to fuel injector drift; an air intake comprising a mass air flow sensor having an inlet and an outlet that generates a MAF sensor output subject to MAF sensor bias; a fuel-air mix sensor having a FAM sensor output; a throttle positioned between the air intake and the engine intake manifold; an ambient pressure sensor and an ambient temperature sensor each having an output; an intake manifold pressure sensor and an IM temperature sensor configured to sense pressure and temperature respectively at the intake manifold; a system monitor configured to calculate each of the following using a model of the engine and the ambient pressure sensor and ambient temperature sensor outputs: a first pressure and a first temperature downstream of the MAF sensor, also using the MAF sensor output; a second pressure and a second temperature at the exhaust manifold, also using a pressure sensed by the IM pressure sensor, a temperature sensed by the IM temperature sensor, engine speed and the mass of fuel; a third pressure and a third temperature at the FAM sensor, also using the second pressure and the second temperature at the exhaust manifold; a controller configured to calculate each of fuel injector drift and MAF sensor bias using the IM pressure and IM temperature, the MAF sensor output, the FAM sensor output, the mass of fue1, the first, second and third pressures, and the first, second, and third temperatures. an engine comprising at least one cylinder, at least one fuel injector, an intake manifold and an exhaust manifold, wherein the at least one fuel injector is configured to inject a mass of fuel to the cylinder, subject to fuel injector drift; an air intake comprising a mass air flow sensor having an inlet and an outlet that generates a MAF sensor output subject to MAF sensor bias; a fuel-air mix sensor having a FAM sensor output; a throttle positioned between the air intake and the engine intake manifold; an ambient pressure sensor and an ambient temperature sensor each having an output; an intake manifold pressure sensor and an IM temperature sensor configured to sense pressure and temperature respectively at the intake manifold; a system monitor configured to calculate each of the following using a model of the engine and the ambient pressure sensor and ambient temperature sensor outputs: a first pressure and a first temperature downstream of the MAF sensor, also using the MAF sensor output; a second pressure and a second temperature at the exhaust manifold, also using a pressure sensed by the IM pressure sensor, a temperature sensed by the IM temperature sensor, engine speed and the mass of fuel; a third pressure and a third temperature at the FAM sensor, also using the second pressure and the second temperature at the exhaust manifold; a first pressure and a first temperature downstream of the MAF sensor, also using the MAF sensor output; a second pressure and a second temperature at the exhaust manifold, also using a pressure sensed by the IM pressure sensor, a temperature sensed by the IM temperature sensor, engine speed and the mass of fuel; a third pressure and a third temperature at the FAM sensor, also using the second pressure and the second temperature at the exhaust manifold; a controller configured to calculate each of fuel injector drift and MAF sensor bias using the IM pressure and IM temperature, the MAF sensor output, the FAM sensor output, the mass of fue1, the first, second and third pressures, and the first, second, and third temperatures.