Loading ...Your BMW’s or MINI’s oxygen (O2) sensor is like the traffic cop at a busy intersection. In the same way that the officer is controlling the flow of traffic, the O2 sensor has the final say on what is going on within the engine management system. By comparing the oxygen in the air to the oxygen content of the exhaust, the O2 sensor can tell if the fuel mixture is too rich (too much fuel, not enough air). If so, it signals the Engine Control Unit (ECU) to reduce the amount of fuel being sent to the cylinders. If the mixture is too lean, the ECU will increase the amount of fuel sent.
Since the O2 sensor is positioned directly in the engine’s exhaust stream, it is exposed to all of the heat and harsh chemicals in the exhaust. Even on a properly running engine, the sensor will, over time, become coated with exhaust byproducts, which reduce its ability to perform. As the sensor becomes coated, its ability to sense changes in the exhaust’s oxygen content deteriorates… right up to the point where it totally ceases to deliver information to the ECU. Without this information, the ECU cannot run the engine at peak efficiency. The result is poor fuel economy and reduced performance.
Replacing the O2 sensor on your BMW is fairly simple. The sensors on most models are readily accessible without removing any additional parts. Let’s take a look at a typical O2 sensor replacement on a 1990 325is. This particular model is accessible from under the hood. On many models, the sensors are located under the car, in the exhaust. NOTE: on a few models, the sensor is mounted on top of the exhaust pipe, requiring that the catalytic converter be disconnected from the exhaust manifold.
1. If required, raise and properly support the front of the vehicle. (Note: The model in this example does not require this because the sensor is accessible from under the hood).
2. Locate the O2 sensor in the exhaust pipe or exhaust manifold (figure 1); rear sensors can be found just behind the catalytic converters.
3. Remove the sensor shield, if equipped (figure 2). The shield in this example clips onto the body of the sensor.
4. Follow the wiring from the sensor to the point where it plugs into the main wiring harness and unplug the sensor’s wire from the main harness (figures 3 & 3A). On sensors that are mounted under the vehicle, It may be necessary to remove some protective plastic shielding in order to fully remove the sensor wire harness.
5. Use the Bavarian Autosport Oxygen Sensor Removal Tool (# OSW1) and a 3/8″ drive ratchet (and extensions if required) to remove the O2 sensor (see figures 4 & 5). We highly recommend using a penetrating solvent such as PB Blaster on the mating area where the sensor meets the pipe. Heat, corrosion and rust will typically have had their way with the threads securing the sensor into the pipe. PB Blaster is available at most auto parts stores and many discount department stores.
6. Install the new O2 sensor (the new sensor will already have some anti-seize compound on the threads). Route the wiring harness into its proper location. Plug the sensor’s harness into the vehicle’s harness. Replace any shielding that was removed, including the shield on the sensor itself.
