A catalytic converter is a part of the exhaust system fitted to most modern petrol engined cars, its primary purpose is to reduce the amount of harmful emissions created by the engine, some of the harmful items a catalytic converter works to decrease are hydrocarbons & carbon monoxide.

This may cause one of the following dashboard lights to illuminate:

A lambda/oxygen sensor is a monitoring device fitted to the exhaust system. Its primary function is to monitor the emissions that your vehicles engine is producing by relaying the exhaust oxygen content/fuel mixture back to the ECU, thus allowing the ECU to make fine adjustments to the fuel/air mixture.

It is not uncommon for a modern car to be fitted with 2 or more Lambda sensors, a conventional layout will comprise a Pre-cat & a Post-cat lambda probe, the one mounted before the catalytic converter is designed to make fuel/air ratio & mixture adjustments & the Lambda mounted after the catalytic converter is there to ensure the catalytic converter is operating efficiently.

See Computer Diagnostics for information on how to resolve emissions faults.