How the Oxygen Sensor Works

Your BMW probably has two oxygen sensors, but might have as few as one or as many as four. These BMW parts operate together to give the onboard computer a clear indication of the composition of the exhaust.

As the exhaust flows over these sensors, they send voltage to the computer. If the oxygen content is high, then the voltage drops. The computer ?knows? that the mixture is lean, so it sends more gas to the engine. If the oxygen content is low, the voltage rises and that means the mixture is too rich. The computer will cut back on fuel flow to correct the problem.

Even when driving a steady speed, the fuel/air mixture will fluctuate as conditions change. When accelerating or decelerating, the mixture changes rapidly. The computer must be able to respond quickly so that the engine runs at peak efficiency and auto emissions are kept to a minimum.

The Problem With Aging Sensors

Like other BMW parts, oxygen sensors don't work as well as they age. The sensor eventually becomes coated with soot and residue from the engine exhaust. This has two effects.

First, the sensor becomes sluggish. It can't react to changes as quickly and it has trouble "seeing" the air through the buildup. That means the computer isn't notified as quickly and fuel flow doesn't change like it should.

Second, even when the sensor does respond, the buildup causes it to see less air than actually exists and therefore reports a lean mixture. This causes the computer to send more fuel to the engine than needed. This not only wastes gas but also causes the car to belch out more harmful emissions.

Over time, the rich fuel mixture will put strain on the catalytic converter. It has to work harder to knock out the contaminants and as it works it heats up. The converter can actually become hot enough to melt the catalyst, causing a blockage in the exhaust and an expensive repair bill.

When Should The Sensor Be Replaced?

Most people learn their oxygen sensor is bad when they fail their emissions test. If you live in an area without emissions tests, you may not find out until serious damage has been done.

These BMW parts should be replaced as part of routine maintenance. The schedule ranges anywhere from every 30,000 miles for the oldest technologies to 100,000 miles for the most current. Check your manual or talk to your mechanic.

A car with a bad oxygen sensor can burn 10-15% more gas than normal and that adds up. The cost of replacement will be made up in no time by your increased fuel efficiency.