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Oxygen sensor is mandatory in application of catalyst to optimize exhaust. When air-fuel ratio of mixed air is off 14.7 to 1, the purification ability of catalyst to CO, HC, and NOX will decrease, so oxygen sensors are installed inside exhaust pipe to check the concentration of oxygen, and send the feedback signal to ECU, then ECU will control petrol injection to make sure the air-fuel ratio is near 14.7 to 1.
Usually there are two oxygen sensors installed, one is in front of catalyst called upstream oxygen sensor, and the other is at the back of catalyst called downstream oxygen sensor. Upstream oxygen sensor checks the concentration of oxygen in exhaust and send the signal to ECU, ECU will then adjust dosage of petrol injection; downstream oxygen sensor also checks the concentration of oxygen after catalyst, and send the signal to ECU, ECU will then compare the two signals sent from upstream sensor and downstream sensor, normally, the signal from upstream sensor should be higher than that of downstream sensor, if the two signals are the same, that means the catalyst may have failed.