Air-fuel Ratio

For petrol the chemically ideal (stoichiometric) ratio is about 14.7:1. This is lambda 1. Diesels always run with excess air, so their ratio is much leaner. Catalytic converters on petrol engines need the ratio to stay very close to lambda 1.

Oxygen sensors measure the exhaust and the engine control unit adjusts injection to hold the target ratio.

A wrong ratio raises emissions and fuel use and can cause fault codes such as lean or rich mixture. Causes include air leaks, faulty sensors, injectors or fuel pressure problems.