Catalytic converter
Definition
The three-way catalytic converter uses platinum, palladium and rhodium coated on a ceramic or metal substrate to convert CO, HC and NOx into CO2, water and nitrogen; it needs the mixture to oscillate around lambda 1 and a temperature above roughly 300 °C. The ECU judges its efficiency by comparing the downstream lambda sensor's activity to the upstream sensor: a healthy cat shows a lazy, stable rear signal. Misfires, oil or coolant burning and excessively rich running overheat and melt the substrate, while physical shock can break it so the ceramic rattles in the shell. A blocked cat causes severe power loss, glowing exhaust and high back pressure; a worn one just fails emissions and sets efficiency codes. Codes: P0420 (bank 1 efficiency), P0430 (bank 2), P0421/P0431 warm-up efficiency; always fix misfires and lambda faults before replacing the cat.