So last month I got an oil change at my grandmas usual mechanic. I’ve never take my Prius there before. I usually check my oil like twice a month, but I put it off and hadn’t checked it until today. Well the oil was overfilled. Not by much, but it burns oil so it could’ve been significant at the time of the oil change, maybe a half quart of excess oil. Interesting thing about this whole situation is that the check engine light came on the day after he changed the oil. Code P0420 for the catalytic converter. Is it pure coincidence that this light came on the day after the oil change and overfill? Right before the oil change, I had just arrived back in my hometown after a cross country road trip. The Prius did great the whole way and the gas mileage was A-okay. Now, the Prius is only getting 30ish miles to the gallon. The check engine light has since turned off. And has gone on and off again one other time.
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.
Yeah it’s got 255700 miles on it, so I’m sure the cat converter was already failing. But the fact that the light came on the DAY AFTER the oil change is what’s got me spinning circles in my head. I guess I was hopeful I could blame this on the mechanic and get a new cat converter at a discounted rate
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.