MECHANIC HELP - P0420

No you just got lucky next inspection you're going to need your cat dealt with or maybe some chemicals poured in it but that's just a stopgap measure possibly for the next inspection

 

FWIW 0420 indicates a Cat Convert Oxygen sensor error.
Coolant level or bubbles would not cause this code.
First, clear the code and see if it resets, it may be Murphy's Law/ coincidence and there is no issue.
Second, look to see if you damaged the wiring during your coolant change.

It may well be, as the Prius is stock, that the sensor has failed and you need a replacement.
The worst case is that the cat has failed by time or some fuel additive contamination.
I don't believe an exhaust leak will cause this code.

A workaround is to buy a spacer to raise the sensor.
Good luck.

 

I believe that code is the catalyst is below the threshold has nothing to do with the sensor of course the sensor is reading that and that's how it knows that the converter can't pass the flood test that the car gives it whenever it says it does and that's that You can change O2 sensors all you want I don't think that will clear your catalyst below threshold code but you're welcome to try spacing out that downstream O2 sensor I don't think will fix the problem it may make the sensor not read as well and not code and that may get you by where you are it may not

 

Sometimes a bad tank of gas can cause this...

 

No, it doesn't. It means the test of the catalytic converter does not meet the threshold to pass the test. While it rarely can mean a problem with the sensor, it usually doesn't.

A mistaken belief. A P0420 can definitely be from an exhaust leak upstream of the sensor.