Replacing the Lambda Sensor or Upper Oxygen Sensor

As you know, the oxygen sensor in modern cars determines from the exhaust gases whether the engine has a rich or lean air/fuel mixture so that the ECU can intervene for optimal engine performance. In this case, we have two oxygen sensors, sometimes referred to as upper (just above the exhaust manifold) and lower (further down, identifiable by following the green cable as shown in the video). Now, let's replace the upper sensor. Follow the cable that branches from the sensor and you will see that the connection is just below the 'U' pipe between the battery and the engine block. Disconnect the connector now. Next, disconnect any springs or clips that support the cable along its path to the sensor. Now proceed to loosen the sensor with the appropriate socket wrench (designed with a slot to avoid damaging the cable) and unscrew it until it is completely removed. Usually, new sensors already have special paste (anti-corrosive, high-temperature resistant, etc.) around the thread, but you can add some if you wish, without overdoing it. Now insert the new sensor by hand and then tighten it with the appropriate wrench. The connector may have a different spring depending on the type of connection, which can be removed if necessary. Before connecting the sensor, it is always best to disconnect the negative terminal of the battery. After reconnecting everything, check if the dashboard displays the error code '421' again. If so, replacing only the upper sensor was sufficient. In other cases, it may be necessary to replace both sensors. Car model: Mazda 6 Year: 2009
Required tools: Socket corresponding to the sensor nut with a slot for the electrical cable, ratchet wrench.