The multigas analyzer simultaneously measures the concentrations of CO, CH4 and CO2 in flowing samples of automobile exhaust. Three channels share a single rotating gas correlation cell and measure the individual concentrations of CO and CH4. Two CO channels with different sample cell lengths extend the useful range. A rotating dual-filter system measures the CO2 concentrations. As many as three separate CO2 channels can share a rotating filter assembly. A system employing two fixed filters and two detectors measures the concentration of H2O and automatically accounts for a slight interference by this gas in the CH4 channel. Discrimination against other typical exhaust gases is excellent. An automatic gain control minimizes calibration drift. The ranges of measurable concentrations are 0.3 ppm (parts-per-million) to 3000 ppm of CH4; 0.7 ppm to 10% of CO; and 60 ppm to 30% of CO2, where the minimum concentrations correspond to the peak-to-peak noise levels with one-second time constants. The theory of operation, the measurement capabilities, and the physical characteristics are described.