P0420 Diagnosis with Freeze Frame Data

Most likely, you need to replace the catalytic converter.

It is true that O2 sensors would produce a lower voltage than the readings shown in your photos. However the first (upstream) sensor is not an O2 sensor. It is an air/fuel ratio sensor and the 3+V reading is normal.

 

The other three codes are meaningless, and the last one, although a legitimate code, may be meaningless as well since your code reader is giving you bogus results. Get Mini VCI to confirm that P0420 is still present and a legitimate code.

The engine ECU is testing the catalytic converter on a on-going basis by monitoring the signals provided by the air/fuel ratio sensor and the downstream O2 sensor, while the engine is in closed loop air/fuel ratio control. The air/fuel ratio is swinging up and down while the fuel management is in closed loop, and the air/fuel ratio sensor readings should fluctuate accordingly.

If you don't believe the results of its testing, which is indicating that the cc is bad, then the only other possibility is that the air/fuel ratio sensor is getting marginal and the voltage swings produced by the sensor are less than the engine ECU would like to see.

So you can try replacing that sensor first, if you like. Use only the correct Toyota replacement part.