Toyota Repair: Error Code P0420

Question
Hello. I have a '99 Tacoma SR5 xtra cab, 4 cylinder, 2.4 liter, manual trans truck. It has about 175k miles on it. The P0420 code has been thrown several times within the past year/8k miles. During that time, I've replaced the rear oxygen sensor and have tried fuel additives like "Guaranteed To Pass" and "Cata Clean." Except for the "Cata Clean", the other two items have cause the light to turn off, but then it's come back on after a few months. The "Cata Clean" had no affect. The light has even gone off, then come back on by itself without anything being done by me. When the check engine light comes on, it tends to do so while I'm driving on the freeway in relatively low RPMs. In your experience, what's the likely cause here? I assume the catalytic converter, but I'd like to know a little more before spending a couple hundred dollars to replace it. Thanks in advance.

Answer
Hello, the P0420 code is very specific to the catalytic converter, it is below the threshold of normal functionality, in other words it is degraded to the point where it doesn't reduce emission within the computer's set parameters, this can be verified by using a diagnostic scantool that can read live data from the #1 and #2 O2 sensors, the #1 sensor is usally called an air/fuel ratio sensor. Observing the voltage patterns using a graph on the diagnostic tool the a/fr sensor should be at a voltage of around 3.25 DC, the rear O2 sensor should show an even voltage pattern of around 0.1 volt or so with almost no fluctuations, if there is a fluctuating wave pattern or if the voltage is higher or lower by as much as 5 volts the catalytic converter is defective.