no idea. i plugged mine in, and it said something like

P0135- heated primary oxygen sensor heater circuit malfunction

(or something like that)

does the manual tell how to do a P0420 test?

it's a bad cat. common at this mileage. just replace it. i see them
all the time on this model.
          Chip

OK, Here it goes 02 class 101 (and catalyst efficiency)

The O2 sensor generates its own voltage, and without my meter I couldn't
tell you, BUT trust your scanner if it gives you a voltage reading. The
first o2 sensor switches when the engines running, you watch it go low to
high (sometimes .020 volts to a high of .900 volts (Milivolts??)
It should click along at a steady pace, picking random numbers into the
700-900 range then low again in the 100's
It is looking for Oxygen. This sensors job is to tune the car if it starts
to run to rich; i.e., if too much fuel starts feeding into the cylinders,
for whatever reason, the o2 sensor will start showing a higher range of
numbers, and the computer will start decreasing the injector pulse to
decrease the amount of fuel.
Anyway, a 02 sensor can get sluggish or it can freeze at a random number
then it lies to the computer that will still try to correct it. but usually
this sets a 02 code...

Now that basically describes the 02 sensors operation. The 2nd o2 sensor's
job is to check to see if the cat converter is doing it's job (Generally,
most 02 sensors are the exact same part excepting for the plug and wire
length). It also switches the same as the first one but at a slower or
reduced rate. If your scan tool shows the aft 02 sensor stuck, I'd try it
first.
If your second (aft) 02 sensor is switching at the same pace as the first o2
sensor (the numbers won't match) then chances are the cat is bad.