Any failure resulting in an over-rich (too much fuel) mixture can damage the catalytic converter. After a repair for the over-rich condition is made, it is recommended the engine oil and filter changed. When an engine runs too rich, the excess unburned fuel gets into the oil and contaminates it. Not changing the oil when it has been saturated with fuel can cause the over-rich condition to continue, making it appear the original problem was not corrected.