Post cat oxygen sensor

420 is a bad catalytic converter, either there is an air leak prior to converter, O2 sensor bad or converter. Most of time it's a converter especially if higher mileage.

Here is an article on to replace the cat

https://www.2carpros.com/articles/catalytic-converter-replacement

If there are no upstream vacuum leaks, most likely the cat is bad, sorry to but in Harry :-)

Exhaust side, if it was intake side you would have a lean code, not a cat inefficacy code. Here is more information on that

https://www.2carpros.com/topics/trouble-codes/p-codes-powertrain/p0400-p0499/p0420

Scan for codes and check fuel pressure with a gauge auto parts rent it.

That info is good, its telling you you have a cat converter that is weak, this is a common problem

here is more information on how to replace it

https://www.2carpros.com/articles/catalytic-converter-replacement

There aren't any instructions to post. You unscrew the old one and screw the new one in.

Possible causes
- Faulty HO2S-11 Sensor
- HO2S-11 Sensor harness is open or shorted
- HO2S-11 Sensor circuit poor electrical connection

Scanners are a smart diagnostic tool but can be very dumb at the same time, yes it is saying that there is a problem at the O2 sensor, but you have to look a little deeper into the problem this code is for an open circuit or poor connection and not necessarily a faulty sensor, so you need to check all connections for loose or corroded connections and do a pin out from the sensor back to the ECU to look for open circuit or high resistance, start here.

OK, good luck with this, these can be tricky to work out, re post if you have any furtjer questions.