Possible clogged catalytic converter issue

My car won't shift down when at a stop until I reach 50 MPH or 3.5 RPMs. I suspected my transmission was going out, so I took it to Kia. They said nothing showed on the diagnostic and suggested it may be the catalytic converters clogged. I see getting these replaced is expensive so is there any way to tell for sure if this is the issue? Also, does my car have one or two? Thanks