We report on an optical system for the detection of carbon monoxide (CO) and nitride oxide (NO) concentrations in the exhaust manifolds of internal combustion engines. In this system a detector based on gallium nitride (GaN) and an UV arc discharge lamp are integrated. The arc discharge lamp induces electronic transitions in the gas molecules flowing between the light source and the GaN photodetector. These transitions modify the fraction of light in the UV spectral region which is detected by the GaN photodetector, as a function of the exhaust gas concentration. The system is designed to operate at temperatures as high as 600 C, so that deposited organic residuals on the detector can be oxidized, ensuring the clear surface needed for an optical measurement.