Petrol car fails emissions test: inspection of the lambda sensor, catalytic converter and fuel mixture in Espoo

A petrol car often fails an emissions test because the engine’s air-fuel mixture control, ignition or exhaust gas purification system is not functioning correctly at the time of the test. However, a high reading alone does not necessarily indicate that the lambda sensor or catalytic converter is faulty. Our investigation begins with the test results, the car’s symptoms and fault diagnostics, to ensure that parts are not replaced based on guesswork. We serve motorists from Espoo at our Kirkkonummi branch.

A petrol car fails its emissions test: what the CO, HC and lambda values tell us

The CO reading indicates the level of carbon monoxide in the exhaust gas, whilst the HC reading indicates unburned hydrocarbons. A high CO reading may indicate an excessively rich fuel mixture or a situation where the catalytic converter is not cleaning the exhaust gas as expected. A high HC reading may be linked, for example, to idling problems, an ignition fault or incomplete combustion. The readings must always be interpreted together, as the same measurement result can be caused by different faults.

The lambda value helps to assess the air-to-fuel ratio at the time of measurement. In cars fitted with a three-way catalytic converter, the emissions test during the MOT examines the CO, HC and lambda values in accordance with the vehicle’s specifications. The limit values and testing methods vary depending on when the car was first registered and its technical specifications, so it is worth checking these. Traficom’s assessment criteria.

The emission measurement value indicates the cause, but the measurements themselves reveal the actual fault.

Petrol car fails emissions test: the lambda sensor isn’t the only suspect

The lambda sensor transmits information on the oxygen content of the exhaust gas to the engine control unit, but a measurement deviation may also be caused by other factors. A leak in the intake system, a leak in the exhaust system upstream of the sensor, a fault in the fuel supply or a misfire can alter the air-fuel mixture. Therefore, a fault code or an emissions test report alone is not sufficient to determine which component needs replacing.

We read the fault codes and the engine’s current readings, compare them with the symptoms, and carry out any necessary further tests. If, for example, the car runs unevenly when warm, jerks during acceleration or fuel consumption has increased, we’ll also check the ignition, intake leaks and fuel mixture settings. In fault diagnosis Our aim is to identify the root cause of the fault before carrying out any repairs.

The condition of the catalytic converter is checked by taking measurements

A catalytic converter reduces harmful compounds in exhaust gases, but its performance should not be assessed in isolation from the engine’s operation. If the engine is constantly delivering the wrong fuel-air mixture or experiencing misfires, even a functioning catalytic converter may produce poor results in an emissions test. We therefore first rectify any running or mixture-related faults and only then assess the condition of the catalytic converter based on the test data.

We’ll also take the car’s service history and current symptoms into account during the inspection. When booking your appointment, please let us know if the car failed its MOT, if the engine warning light is on, and whether there is any jerking or unusual fuel consumption. We’ll carry out diagnostics and any necessary repairs during the same visit workshop services. This ensures that the investigation into the emission fault proceeds in an orderly manner without unnecessary component replacements.