For petrol vehicles, the MOT test focuses on Carbon Monoxide (CO), Hydrocarbons (HC), and the Lambda (λ) value. CO indicates incomplete combustion (rich), HC indicates unburned fuel (misfire), and Lambda indicates the ratio of air to fuel. A healthy modern car should maintain Lambda ~1.00 at all times during the test.

Lambda 1.0 represents the 'Stoichiometric' point—the perfect ratio where all fuel and air are consumed (14.7 parts air to 1 part fuel for petrol). Lambda > 1.0 is 'Lean' (excess air), and Lambda < 1.0 is 'Rich' (excess fuel). Small deviations can often be corrected by the ECU, but large errors indicate physical part failure.

A standard oxygen sensor generates its own voltage based on the oxygen differential between the exhaust and outside air. In closed-loop operation, the voltage should 'flip-flop' rapidly between 0.1V and 0.9V as the ECU constantly adjusts fuel trims to stay at the ideal 14.7:1 ratio. A static or slow-reacting voltage usually indicates a fouled or 'lazy' sensor.

Hydrocarbons are literally raw fuel. If HC is high, it means fuel entered the engine but didn't burn. This is almost always an ignition issue (bad plugs, weak coils) or a 'lean misfire' where there was so little fuel that the spark couldn't catch. High HC paired with high CO, however, usually points to a severely rich condition.