Trouble with these things is that many things will flag that code p420, so its process of elimination ie smoke test proves no unmetered air, then chances are very high its a mechanical issue ie timing related very common, or could be its been using so much oil the cat is now clogged with the crud and thats very very common on them, but going through the process, ie cheaper to test something than replace it, a lot of the time its a number of worn things,, i get them all got one in at mo after refresh, owner put new cat on it never done 02 sensors so we had heater circuit fault code and electronic management fault code, sometimes its just a 02 sensor open short circuit, but on this one it went a stage further and suspect burnt out a driver in the ecu, so ecu been sent off to ECUTESTING.COM found get a good service and life time warranty on repairs from them, and trouble is these engines must run right to stay together,, as running lea