if you 've got a fault light on then by far the best casue of action is to have a scan tool plugged int the car to read off the stored code and so point to the fault
this will save a lot of time in the long run
however anti-polution faults are often an issue with the oxygen sensors in the exhaust
1st Check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.
this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst
The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers
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