A diagnostic system of an opposed-piston engine, the engine including at least one cylinder with piston-controlled exhaust and intake ports near respective ends of the cylinder, a charge air channel to provide charge air to at least one intake port of the engine, and a supercharger having an inlet and an outlet in the charge air channel to provide compressed charge air to the piston-controlled intake ports of the opposed-piston engine, characterized by: a first gas pressure sensor for generating a first signal indicating a supercharger inlet charge air pressure ( P in) in the charge air channel; a second gas pressure sensor for generating a second signal indicating a supercharger outlet charge air pressure ( P out) in the charge air channel; and, an engine control unit in data signal communication with the first and second gas pressure sensors, and comprising means for diagnosing operation of the supercharger by: determining a first supercharger mass flow value based upon the ratio ( P out / P in); determining a second supercharger mass flow value based upon a value of mass air flow into the charge air channel; comparing the first and second supercharger mass flow values to determine a mass flow comparison value; and, if the mass flow comparison value meets a mass flow performance measure, correlating the mass flow comparison value with the diagnostic conclusion that the supercharger is operable to provide compressed charge air to the piston-controlled intake ports of the opposed-piston engine; otherwise, correlating the mass flow comparison value with the diagnostic conclusion that the supercharger is not operable to provide compressed charge air to the piston-controlled intake ports of the opposed-piston engine. a first gas pressure sensor for generating a first signal indicating a supercharger inlet charge air pressure ( P in) in the charge air channel; a second gas pressure sensor for generating a second signal indicating a supercharger outlet charge air pressure ( P out) in the charge air channel; and, an engine control unit in data signal communication with the first and second gas pressure sensors, and comprising means for diagnosing operation of the supercharger by: determining a first supercharger mass flow value based upon the ratio ( P out / P in); determining a second supercharger mass flow value based upon a value of mass air flow into the charge air channel; comparing the first and second supercharger mass flow values to determine a mass flow comparison value; and, if the mass flow comparison value meets a mass flow performance measure, correlating the mass flow comparison value with the diagnostic conclusion that the supercharger is operable to provide compressed charge air to the piston-controlled intake ports of the opposed-piston engine; otherwise, determining a first supercharger mass flow value based upon the ratio ( P out / P in); determining a second supercharger mass flow value based upon a value of mass air flow into the charge air channel; comparing the first and second supercharger mass flow values to determine a mass flow comparison value; and, if the mass flow comparison value meets a mass flow performance measure, correlating the mass flow comparison value with the diagnostic conclusion that the supercharger is operable to provide compressed charge air to the piston-controlled intake ports of the opposed-piston engine; otherwise, correlating the mass flow comparison value with the diagnostic conclusion that the supercharger is not operable to provide compressed charge air to the piston-controlled intake ports of the opposed-piston engine.