Car went from being very clean to failing emissons in the last 20K miles.
Two years ago, had the dealer replace the oxygen sensor, put new plugs, air filter, pcv valve and this brought it into passing. This made it pass, but it was no where near as clean as the previous test cycle. The catalytic converter was replaced 25K miles ago at the dealer with oem new stuff. All the emissions stuff is connected.
Car has 200k miles. Doesn't burn much oil, but does leak some oil down the valve stems.
I don't think there are any vacuum leaks.
Car doesn't idle quite as nicely as it used to.
What is the most likely suspect causing emissions failure on this car?
Thanks