An exhaust gas analyzer or exhaust CO analyzer is an instrument used to measure, in the exhaust of a car, the amount of carbon monoxide and other gases, caused by improper combustion. The Lambda coefficient measurement is the most common.

The main CO (and other types of gases) sensors used are non-dispersive infrared (NDIR) sensors and chemical gas sensors. Carbon monoxide sensors are usually used to evaluate the amount of CO in an MOT. To be used in this test, an analyzer must be approved and approved for use by the competent authority. In the United Kingdom a list of exhaust gas analyzers officially accepted for use in MOT testing appears on the Department for Transport website.[1]​.

The presence of oxygen in the exhaust gases indicates that the combustion of the mixture was not perfect, resulting in the generation of polluting gases. Therefore, by measuring the proportion of oxygen in the exhaust gases of these engines, these emissions can be controlled and measured. This measurement takes place in the ITV test by measuring the Lambda coefficient.

The Lambda coefficient") (λ) is obtained from the ratio of the air involved in the combustion of the gasoline mixture. It is in fact a measure of the efficiency of the gasoline engine by measuring the percentage of oxygen in the exhaust.

When gasoline engines operate with a stoichiometric mixture of 14.7:1 the value of LAMBDA (λ) is "1".

Mixing ratio = weight of fuel / weight of air.

With this relationship, complete combustion of gasoline is theoretically achieved and greenhouse gas emissions would be minimal. The coefficient is defined as Lambda coefficient.

-If Lambda > 1 = lean mixture, excess air.

-If Lambda < 1 = rich mixture, excess gasoline.

NDIR sensors are spectroscopic sensors used to detect CO in a gaseous environment due to its characteristic absorption. The key components are an infrared source, a light tube, an interference (wavelength) filter, and an infrared detector. The gas is pumped (or diffused) into the light tube, and the electronics measure the absorption of the characteristic wavelength of light. NDIR sensors are often used to measure carbon monoxide.[3]​ The best of them have sensitivities of 20-50 PPM.[3]​.

Most CO sensors are fully calibrated before leaving the factory. Over time, the zero point of the sensor must be calibrated to maintain the long-term stability of the sensor.[4] There are new developments that include using microelectromechanical systems to lower the costs of this sensor and be able to create smaller devices. In the US, typical NDIR Sensors are in the $100 to $1000 range.