The main problem is that anti pollution regulations require a sensor after the catalytic converter to check the correct functioning of the cat. The alteration or removal of the cat and/or the post cat sonda lambda sensor is contrary to the regulations in most countries. If the sensor detects an anomaly that lasts for more than a short period (once the engine has reached operating temperature) the engine warning light come on and stays on, in theory, until a workshop has checked the car. Fortunately with the help of an OBD II reader we can check the error code and re-set it ourselves. Obviously if it is not a "harmless" error it should not be cancelled. Some error codes will re-set themselves when the condition no longer exists or the engine is turned off and re-started and others, such as the "malfunctioning" of the cat will stay.

For correct functioning of the cat it needs to get extremely hot and this requires it to be fairly near to the engine. Sports exhaust systems, that are designed to optimise the flow of exhaust gasses, have longer exhaust manifold tubes and this often means it is difficult to raise the cat to its working temperature.

In the absence of an OBD II reader, if absolutely certain it is just a problem with the sonda lambda sensor the ECU can usually be re-set by disconnecting the battery. Most modern ECUs have a "learning" function which adjusts somewhat to the driving style of the driver so it may take a few miles before it completes this process of calibration following a complete power off re-set.

Peter

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