Oxygen Sensors — The Emission Watchdogs

Oxygen sensors are the PCM's eyes into the exhaust stream. They measure how much oxygen is in the exhaust and report it back to the PCM as a voltage signal. The PCM uses this information to continuously adjust the fuel mixture to maintain the ideal ratio of 14.7 parts air to 1 part fuel — called stoichiometric ratio. At this ratio the catalytic converter operates at maximum efficiency.

Upstream oxygen sensors — mounted before the catalytic converter — are the primary fuel control sensors. The PCM reads the upstream sensor signal and adjusts injector pulse width to keep the mixture bouncing between slightly rich and slightly lean. This continuous adjustment is called closed loop operation. A healthy upstream sensor switches between rich and lean several times per second. A sensor that switches slowly — called a lazy sensor — causes the fuel corrections to lag and the engine runs less efficiently.

Downstream sensors — after the converter — monitor converter efficiency. If the converter is working correctly, it processes the exhaust gases so thoroughly that the downstream sensor sees a nearly steady voltage with minimal switching. If the downstream sensor starts switching rapidly like the upstream sensor — the converter is not doing its job. A P0420 or P0430 catalyst efficiency code is set. Before replacing the converter, always verify the engine is running correctly — misfires, rich conditions, and oil burning destroy converters. Fix the engine first.