What does it mean?
OBD II fault code P0432 is a generic code that is defined as “Main Catalyst Efficiency Below Threshold -Bank 2”, and is set when the PCM (Powertrain Control Module) detects that the efficiency of the catalytic converter on Bank 2 has fallen below a minimum allowable level. On engines with two cylinder heads, “Bank 2” refers to the bank of cylinders that does not contain cylinder #1.
The purpose of a catalytic converter is to reduce harmful exhaust emissions to less noxious substances through a process of oxidization that depends on a variety of precious metals in the converter being at very high temperatures. When the exhaust stream passes over or through the precious metals, aka, the catalysts, a complex series of chemical actions and reactions convert especially oxides of nitrogen into harmless substances like oxygen and water.
However, while catalysts are not consumed in the conversion process, there are many factors relating to normal (or sometimes abnormal) engine operation that can either shorten the useful life of a catalytic converter, or damage it to the point where it no longer works as intended, with serious negative effects on fuel economy and the environment.
Therefore, to monitor the efficiency of the catalytic converter(s) on any application, the PCM uses input data from (among other sensors), both the upstream and downstream oxygen sensors (or on some applications, the air/fuel ratio sensors).
In a fully functional system, readings from the upstream oxygen sensor fluctuate greatly several times per second because it reacts to changes in the oxygen content of the exhaust stream, while readings from the downstream sensor remain fairly constant, provided the catalytic converter is operating at a high level of efficiency.
In practice, the PCM compares the input data it receives from the up -, and downstream sensors continuously, and if it detects that the readings from the downstream sensor matches those from the upstream sensor too closely, it uses the compared input data to calculate an efficiency level for the affected catalytic converter. If this calculated value falls below a preset minimum value, the PCM will set code P0432, and illuminate a warning light as a result.