AFR

📖 YOUCANIC Automotive Glossary

AFR (Air/Fuel Ratio) is the mass ratio of air to fuel. Stoichiometric for gasoline is 14.7:1. Higher ratios (16:1) are lean; lower (12:1) are rich. The ECU maintains stoichiometric during closed-loop operation but commands richer during cold start and WOT, and leaner during light-load cruising on some engines.

Some vehicles use wideband AFR sensors providing precise linear readings across the full range, rather than narrowband O2 sensors’ simple switching signal. Wideband sensors display actual ratio (14.7, 15.2, etc.) rather than 0.1-0.9V switching. They should show stable ~14.7 during steady-state cruising, not the constant switching expected from narrowband sensors. Wideband sensors are more expensive and use different diagnostic approaches.