Hello, to answer this question it is helpful to get a basic understanding of what the catalytic converter is doing and what the testing is showing.
The catalytic converter's job is to take exhaust gases from the engine and "clean it up" before coming out the other side and leaving through the muffler and tail pipe. There are precious metals inside that help accomplish this, and as the converter reaches extremely high temperatures (starting at over 400 degrees Fahrenheit, climbing to sometimes over 2,000 degrees Fahrenheit). If you have excessive hydrocarbons in your exhaust -- such as unburnt fuel due to an engine performance problem, the temperatures can rise greatly. If they rise greatly, the internal components will become damaged and ineffective.
The way this is measured by a sensor before the catalytic converter and a sensor after the catalytic converter. Both are exhaust gas oxygen sensors, measuring the content of the exhaust gases before and after treatment. If there is no evident change, the check engine light will come on and set a "catalyst inefficiency code", typically P0420 (and/or P0430 for cars with V6 or V8 engines). Keep in mind that this code alone does not condemn the catalytic converter. Software issues could cause this, as well as a concern with one of the sensors or wiring. However, in the majority of cases, the sensors are reading properly and the converter has become ineffective.
All that to say this: If you have verified that your catalytic converter is not effective at performing it's job: there is no other answer but to replace it. There is no "cleaner". It is what it is. Once it has failed, it has failed, and must be replaced.
Definitely follow through to ensure proper testing has condemned the component and that it is nothing "simpler" and act accordingly.
