2009 Mechanic Overfilled the Engine Oil

Yep... Prius hold less oil than most cars and mechanic didn't know... While it can hold 4 quarts on an empty brand new engine, usually 3-1/2 quarts is how much I add on an old engine to get it to the full line on dipstick because it's hard to get all the oil out once you put it in. Your low MPG and error code for a catalytic converter is proof that they made a mistake. Document everything in writing with as much information as possible and tell the mechanic they're going to do another oil change with correct amount oil and they're going to replace your catalytic converter or all the documents get submitted to small claims court, which will be more expensive, not less expensive.

 

You don't have much information about the condition of the OPs car to be threatening a mechanic over an oil change overfill. We have no idea if it was overfilled by 1/4 quart or a full quart. A subjective measurement by the OP with a car that's over 10 years old and unknown amount of miles. To be threatening a mechanic after an oil change is a bit premature at this stage

 

With the OP stating this in his post:

but it burns oil so it could’ve been significant at the time of the oil change, maybe a half quart of excess oil.

I would venture a guess the catalytic converter has been heading in the 'fouled' direction for a while.
15 year old car, no stated miles but stated it's an oil burner. Mechanic was likely just in the wrong place at the wrong time for the wrong oil change.

 

I wouldn't even let it slow me down. If you have a good compressor, you can pull that entire pipe off, plug the muffler end and blow compressed air in through the O2 sensor hole. Air will blow backwards through the pipe and, normally, a huge cloud of black soot comes out the 'inlet' end of the pipe. On another note, aftermarket cats are available (currently about $100) that I've installed in a couple dozen Gen 2s over the last 3 years and have only had one throw a code since installation. I also always install a new Denso O2 sensor from Rock Auto.