How much does a catalic convertor cost

You would really need to have someone monitor the H02 sensors to see if the catalyst has an efficiency problem the two o2 sensors will mirror each other if the cat is on its way out, another way is to physically check and see if the elements are coming apart inside of the exaust. Usually you can tap on the exaust pipe litely with a rubber mallet and you can here peices of it rattling aroung if that were the case