Closed Loop Operation Oxygen Sensor

When in closed loop, the eCm uses the oxygen sensor voltage signal to make minor corrections to the injection duration. This is done to help the catalytic converter operate at peak efficiency.

When the voltage is higher than 450 mV, the air/fuel ratio is judged to be richer than the ideal air/fuel ratio and the amount of fuel injected is reduced at a constant rate. The reduction in the duration continues until the oxygen sensor signal switches to a low voltage (lean air/fuel ratio).

Exhaust Oxygen Content

Oxygen Sensor Output

Air/Fuel Mixture Judged To Be:

High, Above 0.45 volts

-- Rfch

High

Low, Below 0.45 volts

-^ _

When the voltage signal is lower than 450 mV, the air/fuel ratio is judged to leaner than the ideal air/fuel ratio so the amount of fuel injected is increased at a constant rate. The increase in duration continues until the oxygen sensor switches to high voltage (rich air/fuel ratio). At this point, the ECM will slowly decrease the amount of fuel, therefore the air/fuel ratio oscillates slightly richer or leaner from the ideal air/fuel ratio. The result is an average of approximately 14.7: 1. This produces the proper mixture of exhaust gases so that the catalytic converter operates at its most efficient level.

The frequency of this rich/lean cycle depends on exhaust flow volume (engine RPM and load), the oxygen sensor response time, and the fuel control programming. At idle, exhaust flow volume is low, and the switching frequency of the oxygen sensor is low. As engine speed increases, the switching frequency of the oxygen sensor increases, generally eight or more times at 2,500 RPM in ten seconds.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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