so the new tstat u installed with the wp was bad? did u test it on the stove in water? can you still check the old one?
tom

The old t-stat was stuck closed. I complicated things by not knowing the system had to be bled. i replaced the WP as I was in there and it has 130,000 miles.

ANOTHER lesson learned.

Now that I think of it, I believe that I have had more than a few of these come in with codes, and the temperature gauge reading low ( opposite of yours ) , and upon checking things over, finding out that the t stat was stuck open. Hopefully, yours wast just stuck closed.
Does not look like a head gasket would be much fun on one of those. I have never done one, and that leads me to believe that it is not all that frequent. There are gazillions of those motors running around , you would think I would have had one apart by now .
I guess you will know by the end of the day.
I am working on a 2010 sienna van today with the same symptoms as yours . I found a slow water pump leak, and although it was topped up, it has to be bleed to work properly .
I got heat back as soon as I bled the system .
New cars , ugh.......