An optical fibre sensor to measure weak partial pressures of carbon dioxide has been designed and realized. This sensor is based on direct molecular absorption in the near infrared at 4.3 μm. It is an extrinsic and amplitude modulation type sensor. In the linear region between 0 and 2000 μbar, the calibration curve that represents the transmitted power versus the carbon dioxide partial pressure in air shows a sensitivity of 20 nW/mbar with a minimum detectable pressure of 25 μbar. This article describes a temperature sensitivity analysis for all optoelectronic components and proposes a calculation method to distinguish two successive levels of partial pressure in a surrounding atmosphere where the temperature can vary. The experimental results show that without using a reference signal, the maximum variation of temperature is ±1.4 °C. The use of a reference signal combined with thermoelectric cooling of the photodetector tolerates a temperature variation of ±25 °C.