P0430 Jeep - Appearing with other codes

Question

I have a 2005 Jeep Grand Cherokee limited, with a 4.7L V8 engine. Recently the check engine light was on, so my husband and I had it looked at by a mechanic who told us we needed to replace the EGR valve (I forgot the original code). After replacing the valve, the check engine light was off for a few days, but came back on. When it came on, it would last for 1-2 days and then would be off for a while (usually for a week or two). This intermittent CEL continued for several weeks, until we took it back to the mechanic. The mechanic checked the codes again and told us that it was insufficient airflow (P0401), and suggested that he clean out the rest of the EGR system to remove any clog or build-up. Before getting more work done, we decided to get another opinion. The second mechanic told us that it was showing a P0406 and P0430 code. He told us that we needed a new EGR valve (we had not yet told him that it was just replaced), and that it could need a catalytic converter (or the P0430 code could be caused by the P0406 code). After getting this information, we contacted the first mechanic again, who told us that the P0430 code is most likely related to the airflow issue (or it could be a bad 02 sensor). Before throwing a bunch of money at the problem, I would really like to know what the most likely culprit is. Any information and/or feedback would be much appreciated. Thank you!