An optical fibre presented is capable of monitoring the presence of carbon dioxide and carbon monoxide in the exhaust system of petrol engines. At present there exists no commercial sensor, which is capable of providing online measurements of these exhaust gases as required by European legislation. The design of this sensor using low cost and compact mid-infrared components, which make it suitable for insertion into a vehicle, is presented. Results are presented for 200 ppm step changes in concentration of carbon monoxide supplied from a laboratory controlled supply is presented. (C) 2007 Elsevier B.V. All rights reserved.