(quoted from post at 00:49:34 01/05/19) I'll start with a bit of education.
How do oxygen sensors work? The PCM (computer, or whatever you choose to call it) provides a .500volt reference signal to each oxygen sensor. As the sensor is exposed to exiting exhaust gases, it reacts by generating a voltage. Under .500 volts is seen as lean, over that voltage is seen as rich. Every time that sensor voltage changes from over to under the reference voltage, it is called switching. The rate at which this switching occurs is monitored by the PCM. If the sensor stays on the rich or lean side for too long or too often, the PCM generates a mixture code (P017x). Since there is a sensor before the catalyst as well as one after the catalyst, this gives the PCM the ability to monitor the efficiency level of the catalyst by comparing the switching rates of the upstream sensor to the downstream sensor. When the PCM determines that the catalyst is losing its effectiveness, it will generate the P0420 series codes.
In this case, once you have generated that code, it usually means that the catalyst has reached its end of life. Changing oxygen sensors will not likely do much more than cost you money. If the sensor or sensors were bad, they would have already generated a code telling you that. Since they did not generate such a code, changing them will probably not fix the problem.
Next a few words about catalytic converters. A catalyst by definition is a material that alters a chemical reaction without becoming part of that reaction. Automotive catalysts are designed to last the life of the vehicle that they are installed in. They are exposed to very hot and corrosive exhaust gases exiting the engine. The most likely reason for failure is contamination. It is said that the zinc in oils used in flat tappet engine is one source of contamination. Other metals like lead also cause contamination of the catalyst. It is quite likely that if you are able to de-contaminate the catalyst using that cleaner product, it will probably help more than changing sensors.
A word about the "anti fouler" trick. Using anti fouler is done by removing the downstream sensor and installing an anti fouler in between the sensor and the catalyst. The "science" behind it is that it theoretically will slow down the switching rate of the downstream sensor which causes the PCM to see a better efficiency level of the catalyst. I have tried this on several vehicles, but have not had any success in eliminating the P0420 code this way. Your results may vary.
After you used the cleaning material and drove the vehicle over 600 miles without the code returning, the cleaner would appear to have worked. By this mileage, the PCM is apparently satisfied that the problem has been resolved.
Now a word about trouble codes.
There are normally 3 types of codes. Most common is what is called a "hard fault" code. this is a code that is generated by a failure of a circuit or sensor that is always present. An example would be a failed TP sensor. The PCM sees the voltage reading from that sensor to be out of range, and immediately generates a code that will not clear until the hard fault is repaired.
Next are codes generated by "monitors." Monitors are areas of memory in the PCM that store information about the operation of the engine in real world operating conditions. Several functions are monitored and data is stored about the operation of these functions. Among those that are monitored are the oxygen sensor circuits, EGR circuits, evaporative emission circuit, catalytic converter efficiency, and several others that vary by make, model, and equipped options. Codes for these functions are only generated after certain conditions are met when each if the monitors has stored sufficient data. When the data is not within normal limits, a code is generated as in the case of your catalytic converter.
The third type of code is a "memory" code. These codes do not normally cause the "check engine" light to turn on. They are stored as possibly intermittent incidents, and do not generate a fault code until a fault has occurred enough times to satisfy the PCM's programming to cause the MIL (check engine) light to come on.
Last, but not least, I suggest that if you are going to attempt doing your own diagnosis and possibly your own repair work, you should invest in a decent scanner for the job. I got one that is updateable from Auto Zone for under $250 that has a lot of capabilities - including reading sensors live, clearing codes, and a few other functions. With repair shops charging over $100/hour for this type of work, it would pay for itself in very short order - and in the process would save you a pant load of money.
Sorry for such a long post, but I hope you did read through it and learn something useful from it.
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