Denso / NTK Oxygen Sensors, these are the same as OEM Genuine Mazda ones but much cheaper than genuine.
Oxygen (O2) Sensors commonly fail due to contamination. When checking or replacing an O2 Sensor, visually inspect the sensor body to determine if it has been contaminated by a faulty part or substance upstream, and correct the fault before replacing the sensor.
Keywords
Oxygen sensor O2 o2
