What is normal for long-term fuel trim?

A short while ago I picked up a 2005 Prius with roughly 264,000 miles on it and have been working on resolving the issues it had when I got it. After a LOT of cleaning and some minor fixing, I'm down to one code and one (potentially related) issue.

The code is P0420, Catalyst System Not Performing to Efficiency- typically interpreted as 'replace the cat', but not necessarily having been caused by the cat going out. Here's a pretty good write-up about the P0420 and data from the sensors: http://www.easterncatalytic.com/education/tech-tips/the-misleading-nature-of-the-po42o-code/. Admittedly it was written for a normal ICE vehicle so some of it doesn't work quite as well for a hybrid, but the basic operation of the Prius's ICE is the same as a non-hybrid.

As I have a wireless code reader, I've been trying to use the sensor data to determine exactly what is going on- I haven't had the chance to record a full log of a trip with the newer app that I just got, but kept an eye on the readings as I was driving it last night. I need the longer log to be able to determine if the O2 sensors are both responding properly and see trends in the temperatures.

But the one thing that stood out to me was the long-term fuel trim % number: 11.72%

The short-term obviously fluctuates a good bit, and I want to get a full log of data of it too so I can see what it's doing over the course of a drive, but the long-term just looked like it was rather high to me. So- what is the 'normal' long-term fuel trim % for our cars?

The 'issue' I'm noticing which I'm not sure it related but might be is that the ICE seems to shudder or stumble a bit when shutting down either at a stop or in motion due to transition to EV. It does it to what seems to be a lesser extent when it starts, but I have no idea how smooth it should be not having any experience other than with traditional ICE vehicles.

 

That it should be a smooth transition is what I was figuring it should be. I just cleaned the MAF & throttle body earlier in the week and that took care of several other codes that it had been throwing. I have new plugs arriving shortly and will be replacing them (know PO replaced them not too long ago but also have no confidence they got the right kind, and want to be able to inspect them anyway). I haven't done anything with the injectors, and haven't closely checked for throttle body leaks. I'm going to assume that's done the same way as with any other ICE using something like propane to look for changes in idle (though GETTING the engine to sit there and idle while doing this is something I don't know how to do yet...)?

I've looked into the miniVCI but am not quite ready to take the plunge and get it just yet- I'm probably not going to have the car for very long so don't want to get too much in the way of specific tools for working on it

 

OK, so I picked up DashCommand before the weekend and got the chance to do some data logging with it while driving around, and put the data and a few charts into a Google Spreadsheet. It's really pretty interesting- there's a ridiculous amount of data in there, and the charts clearly show the usage of the gas engine vs. the electric motor, and is from my driving around town doing some shopping. The link is here- I've made it (I hope) viewable (and hopefully downloadable if anyone wants to play with it) to everyone: Prius_Data_Log_1 - Google Sheets

It will probably be useful to 'zoom in' on a few of the stretches where the gas engine was being used to see what the response from the sensors is, but I'm far less skilled at manipulating data in Open Office's spreadsheet editor than I am at Word so I haven't gotten around to trying to do that yet.

I will make another run after adding the Techron- but it's worth noting that I'm going to be doing a lot of other work before then (new plugs, replacing PCV valve, etc.) so it may not be the best 'direct' comparison...