Volvo S40 check engine light remains on after replacing O2 sensor, spark plugs, ignition coils, mass air flow sensor, and oil change on Volvo S40

Some Volvo S40 owners report experiencing a persistent check engine light even after replacing the O2 sensor, spark plugs, ignition coils, and mass air flow sensor, as well as performing an oil change. This issue may be attributed to exhaust malfunctions, such as internal faults or broken components in the exhaust system, which can cause excessive emissions and alter the air-fuel mixture. Additionally, owners may face ongoing oxygen sensor failure, despite having replaced the sensor, as it is crucial for monitoring unburnt oxygen in the exhaust. Spark plug issues can also contribute to the warning light remaining illuminated, particularly if there are problems with misfiring or the catalytic converter. Furthermore, a faulty mass air flow sensor can lead to poor engine performance and trigger the check engine light. Lastly, some owners find that the light returns after repairs if the root cause of the problem has not been fully addressed, indicating the need for further diagnosis to identify the exact issue.