A new way to fabricate thick-film semiconductor-type tin oxide (SnOx) carbon monoxide sensors is reported in this paper. The sensing material is made by metallo-organic decomposition and is composed of calcium oxide- and niobium oxide-doped tin oxide with platinum as a catalyst; the electrodes and the heater are made by screen printing. Sensing measurements are done by cycling the sensor temperature with high and low heating powers. The CO response curve is almost linear at CO concentrations below 400 ppm. The sensor can detect CO concentrations below 30 ppm. The CO response with time, CO sensitivity, operation temperatures, power consumption, reliability of the sensor and also the selectivity of the sensor to various gases have been investigated.