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The Curious Bag
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<h4 class="dechead">On One Hand: Verify Sensor Parameters
An oxygen sensor (O2S) that has an output voltage or resistance that is out of manufacturer specifications is a clear indication you should replace it. You can troubleshoot the sensor using a multimeter, following the instructions on the vehicle service manual for your particular BMW model, and compare the readings to those values given on the manual.
On the Other: Good Lazy Sensor
Although a multimeter can show a sensor within manufacturer operating parameters, exhaust byproducts will coat and block the sensor's operating elements after years of service. If a sensor becomes too lazy or slow to react, fuel economy will decrease as emissions increase, affecting engine performance.
Bottom Line
If your tests show an oxygen sensor within manufacturer parameters, do not replace it unless you know it is approaching the 50,000 to 60,000 miles (one- two-wire sensors) or 100,000 miles (three-wire or heated oxygen sensor) of service life.
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