The P0420 came back, but.
New update:
I am not ready to apply for sainthood yet, but I put some SeaFoam in my half full gas tank and ran it down. The p0420 that has been bugging me for several months has completely gone away for about 800 miles. Come to think of it, that's what cleared it the last time. I am hoping it's a very dirty injector.

Seems like maybe I was not paying enough attention to the fuel trim numbers. LTFT was sitting near 0, now -10 seems like normal. Now both LTFT and STFT stay mostly constant while driving, STFT used to vary a bit more.

Finger still crossed!

 

Finger crossing did not help. P0420 is back.

 

gee Don sounds like time for a cat replacement.

 

So, last week I replaced the purge valve and cleared the codes. EVAP finally ran and ALL codes are gone, history, current and permanent.
It takes 17 cold starts to run the EVAP after clearing codes.

 

So no new cat?

 

More than 1500 miles so far. Still crossing fingers. The last few times it failed after the EVAP ran.

Recap: new Evap Solenoid, new Evap Vent, new #1 O2 sensor, new #2 O2 sensor, new flange gasket and a few of aggressive SeaFoam treatments. Cleaned the MAF, found out cheap Bosch sensors don't work on the #2 side, but do on the #1.

 

Well that didn't last long! Popped back up today.

It was 1800 miles between clearing the code and re appearance. About 200 miles after EVAP was run.

 

Well, this is annoying! Put a new cat in, P0420 returned.

A couple of mechanics have suggested a computer glitch. Come to think the P0420 first appeared shortly after Bonzo programed the SDM last August. Grrr another $100.

 

Bonzo programmed your SDM?

Wasn't he in a Ronald Reagan movie?

 

Yes, he was a chimpanzee. I suspect the guy ended up reloading the firmware, and mucked it.