there is an upstream o2 sensor before the catalytic converter and a downstream o2 sensor after the catalytic converter...
both read exhaust emissions and instruct the ecm (electronic control module) on proper air/fuel ratios for the electronic fuel injection system...
a vehicle won’t run properly and the check engine light will constantly be on...
What did engines do before these devices? The world survived for close to 100 years before the nazi’s in amerika took over.