Just did valve job and now it is setting a P0421 code

Question

My daughter has a 2002 Mitsubishi Lancer ES 2.0 4G94. Timing belt broke and was replaced aprox 3 years ago. Amazingly the car ran for 3 years but with compression slowly dropping on #2 cyl. until it got to the point of causing a misfire code. I just completed a valve job and now compression is fine across all 4 and the car runs super again except now it set a Po421 code. The code came on at about 40 miles into the test drive to ready the system for inspection. First it was a pending code with no mil then it set a code po412 and the mil came on, it still has a separate pending po421 as well. This code was not present prior to the valve job and no sensors were replaced and all connectors are tight. The O2 sensor was replaced when the timing belt was replaced. Somehow I must resolve this and money is too tight to start throwing parts at it by guessing! Any suggestions?

Answer 1

For 3 yeas engine was running with low commpresion!  So catalityc was under stress too long, it May be O2 sensors are damaged,catalytic as well or after valve job some vaccum or air leak ? all gessing.