An internal combustion engine ignites a mixture of gasoline and oxygen with a high-voltage spark. Unfortunately, a residual quantity of hydrocarbons, carbon monoxide and nitrogen oxide are sometimes left over after combustion. A catalytic converter breaks down these byproducts and recombines these pollutants to form harmless gases like carbon dioxide (CO2) and water (H2O). Inside the converter, precious metal catalysts beds accelerate these chemical reactions without being consumed by the process itself.
The precious metals used as catalysts inside a converter are distributed over a ceramic honeycomb substrate. Over time, the precious metals can separate from the ceramic elements or become poisoned by substances like phosphorus, zinc and silica. On some vehicles that are running too rich, the catalyst beds can be blocked by carbon deposits. The health of the converter is monitored by oxygen sensors mounted upstream and downstream of the converter. The ECM looks at the information from the oxygen sensors to determine if the catalytic converter is functioning. Basically, it is looking at if oxygen levels decrease after the gases leave the converter. The ECM is continuously looking at the oxygen sensors when they are warm enough to function during open-loop operation. The test or monitor scores the converter using a number called catalyst efficiency. A new converter will be 99% efficient. If the number drops below a programmed threshold like 92%, a code is set, and the check engine light is turned on. This is the dreaded P0420 code for Catalyst System Efficiency Below Threshold for Bank 1, or P0430 for Bank 2.
So what is the cure? The answer is typically upstream. The cause can range from a leaking head gasket to even the wrong oil in the crankcase. It is critical to diagnose these issues before replacing a catalytic converter or even an oxygen sensor. But in some cases, it comes down to the age of the catalytic converter. The longevity and effectiveness of the converter are determined by the amount of precious metals present on the honeycomb substrate.
If you get a P0420 or P0430, first solve problems changing the combustion chamber’s conditions before you start throwing parts at the code.
This video is sponsored by Rislone.