what's the solution for the po420 code,if i just changed the umstream a/f sensor

Hi there: That is the code for the Catalytic Converter, my friend, and you should not have changed the A/F sensor. This code is the "Kiss of Death" for vehicles and if you do not get it replaced, then it will cost you a lot of money down the road. And also some pieces of the converter will get into the exhaust system and that will cost you a lot of money to get it fixed. That is the converter code for sure. So take it to either NTB, Midas or some place like that. DO NOT take it to your dealership since they will charge you a lot of money for to diagnose it and replace it. DO NOT take it to a gas station, but you can take it to a reputable mechanic to get it done right. Have the mechanic do a FULL diagnostic test on the Check Engine light to make sure that it IS the converter. If it is, the cost to get it done SHOULD run you between $700-$900 depending on where you go. If you go to a dealership or a gas station, they will run you close to $2,000 or so. When I took my car today to Midas, they will charge me close to $850 to get it done and to get it done right, compared to the gas station near my house who was going to charge me close to $2,000 for that. Hope this helps.

poss bad catt converter on bank 2.

That means you need a new cat make sure something else is not wrong causing the cat to go bad like a seriously rich condition or something.