A high-precision, high-sensitivity carbon monoxide (CO) sensor was developed for early fire detection. This sensor relied on a continuous wave distributed feedback (DFB) laser emitting at a wavelength of approximately 2.33 μm as an excitation source. A 2f/1f Wavelength Modulation Spectroscopy (WMS) strategy was adopted to isolate complex, overlapping spectral absorption features associated with ambient pressure and to achieve excellent specificity and high detection sensitivity. Allan-Werle deviation analysis was used to evaluate the long-term performance of the CO sensor system. A Limit of Detection (LoD) of 1.18 parts per million by volume (μL/L) was achieved with a measurement precision of 0.08 μL/L for an optimal integration time of ~205 s. The early fire detection of paper, cotton, and pine wood was conducted in the field, which verified the reliable and robust operation of the developed sensor.