Exhaust gas recirculation (EGR) is a method to reduce NOx emissions from vehicle engines. It works by piping a portion of exhaust gases back into the intake manifold, which lowers peak combustion temperatures and reduces NOx formation. Testing showed that EGR significantly reduced NOx emissions from engines in a reliable and fuel-efficient way. While EGR decreases available oxygen and can impact performance, it is an effective technique for controlling harmful emissions and improving engine life.