Success!
Only one code in the ECM; - PO300 - general misfire
Totally agree with the code/diagnosis as this is consistent with delivering the chemical through the PCV vacuum port. This always causes a rough running engine while it clears the chemical and carbon out and settles down to a smoother idle. Usually the engine runs a bit rough, the CEL flashes and then comes on solid, followed by a few restarts afterwards which seemed to clear the code, but not this last time.
A little unsure why the ECM didn't clear once the engine started running on 6 cylinders after the combustion chamber cleaning was finished and the van completed a 250 mile highway run with several starts and stops.
So, finding no other codes, we cleared them and so far it hasn't returned.
Emissions test Wednesday.