Engine Performance problem
1999 BMW 528 6 cyl Two Wheel Drive Automatic 109000 miles
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How many O2 sensors are on this car? How do you replace them. What are the part numbers?

There are four Oxygen sensors on that vehicle. Replacement would depend on the status of the original sensors, in other words, is there a reason to replace them? The two precat sensors are for fuel trim and the the postcat sensors are there to determine the catalytic converters efficiency. I know BMW recommends replacement of the sensors at 100k miles, however, I have seen these sensors doing just fine at much higher mileages. For the part numbers, go to Realoem.com, there are several BMW parts sites that you can visit. Lastly, if you need directions for the replacement of these sensors, I recommend that you purchase a Bentley repair manual, they are available online. The postcat sensors rarely require replacement, the precat sensors are on the exhaust manifold and are accessible from the engine bay on the right side of the car.