Ive read other forums where the SG wasn't accurate upto a 20% difference on a prius II..
bummed out that it isnt as good as I thought it to be, but there are lots of other things that are just as good to use for proper instrumentation..
Had a CEL yesterday code 0420.. I reset that instantly while driving
its a slight exhaust leak from the manifold to the exhaust.. 2 of the 3 bolts were cross threaded from the factory, one came out ok, the other snapped and ones fine..
put it back on, the one that came out ok started getting way too tight (1/4 from tightened right) and so basically only one stud is holding anything on..
So I think thats the situation for the 0420..
It only comes on every 3 - 400 miles..