Well I took my car in to ford and cause the check engine light came on reading that one cylinder is runing lean. I knew I had a bad exhaust leak and they said that was the problem so I bought a new mid pipe with high performance cats to make emissions happy so they are putting that in and he said he thinks it will fix the problem. I didnt think a leak would do that.

 

if the leak was before the o2 sesnors it will give them misleading readings possibly causeing lean codes. because if the exhuast leaks before the 02 sensors they will never read the rich or lean mixture that is present.