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My warning lights came on for abs and brake. I was told this is probably a speed sensor issue, but the lights came on while I was on my way to put air in my front tire that was noticeably low. Can tire pressure affect the speed sensor readings? I hate to have the dealership start replacing the speed sensors at 375 a pop if there is an easy fix. Thanks in advance for any help.
 

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The Speed Sensors for the ABS and Traction Control read the rotation of each wheel assembly and compares them. That said, if you have a tire that is very low in air compared to the others, it is actually rotating at a different speed than the others and the warning light(s) can come on. There are some other things that can cause the warning light(s) to come on including; a damaged sensor wire (did you have any of the wheels off right before the warning light(s) came on? Possible that you damaged the wire or disconnected it), obviously a bad sensor or sensor ground (you can remove each sensor and clean the road dirt and rust from the mounting surface of the sensor and the hub and bolt them back in, this should fix a bad ground situation), a water leak into the passenger compartment that gets either the fuse box and wires damp or at the back of the dash cluster (you can do a complete instrument cluster test to check all gauges and lights, if they all check out during the self test, pull the cluster out and make sure that the feed plugs are all tight and there is no moisture there, check all fuses and make sure that there is no moisture or corrosion there), and lastly, you could have a Speed Sensor that is bad or the Drive Wheel on one of the corners is bad/broken/extremely filled with dirt (can be inspected when you remove each Sensor to clean and check the ground). I had one of my Speed Sensors on my 2003 325xi cause the light(s) to come on and it was the ground between the Sensor and the Hub, after cleaning all 4, no more light(s), they are much more open to road dirt, grime and water than those in the E30s.
 
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