New Upper O-2 Sensor & Red Triangle of Death

my otherwise excellent mechanic shop just replaced the upper O2 sensor on my 2008 Prius. immediately after driving the 4 or 5 miles home, I started the car the next day and the red triangle of death is on! I'm afraid to drive it even back to the mechanic. I'm new to the Prius. And the engine light is back on which pulled the code for a minor vacuum leak. Changed out the gas cap, but haven't gotten any further. what about this death triangle. Plenty of new oil plenty of radiator water, flushed the brake system i'm not a very deep mechanic. I'can do a few little adjustments