Function Explanation (DTC Detection Outline)

The PCM monitors the oxygen storage amount proportional to the catalytic converter purification characteristic.
The PCM starts to monitor after recovery (fuel injection starts) from fuel cut control during deceleration.
The PCM temporarily changes the target air/fuel ratio to rich at recovery from fuel cut control, and detects the response time until the HO2S output signal outputs the voltage on the rich side.
If the catalytic converter purification characteristic deteriorates, the amount of time until the HO2S detects the voltage on the rich side is shorter compared to the normal purification characteristic as shown in the figure.
The PCM detects a catalytic converter malfunction using this characteristic.

Fig 1: A/F Sensor Signal And HO2S Signal Waveform

Courtesy of MAZDA MOTORS CORP.