Q: code PO431 on 2002 Kia Sedona

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
Hello i bought a kia sedona 2002 a month ago and this code (po 431) keeps coming on. they check everything, they changed a transmition sensor and it did work find for a couple of days but the light came back on! the mecanic can not find what's wrong. the oxigen sensors look good, the catalytic too. could you give me an idea of what could be the problem?
(1) Answer
A P0431 means that the Warm up Catalyst on Bank 2 has lost it efficiency and needs to be replaced. Depending on the amount of miles, you may be covered under the 7year/50,000 mile warranty for emission parts that cost more than $400.