M3 - ACS and brake light come on and off for no real reason

[Ian Jarvis]

Just driving along and I look down and the two lights are on. The will stay on then, until I turn engine off and then stay off until some other moment while driving. Last year I got through state inspection with luck - they stayed off. This year I'd like to get through because I've replaced the switch or connection that is clearly malfunctioning. All advice will be gratefully received

[Manolito]

First, look for damage to a wire to a wheel speed sensor, or damage to the sensor, or a sensor that is not the same distance as the other three from the reluctor wheel that it is reading, or damaged teeth on a reluctor wheel.

You will probably save some time if you take it to a shop that has a tool that will read the codes from the ABS/ASC. That should narrow the list of suspects.

[Ian Jarvis]

yeah a wide range - maybe it is a trip t the shop for some diagnosis - thanks for both replies. Much appreciated.

[bostogrun]

The traction control share a lot components. The way traction control works is when one is moving faster than the other the traction control activates a brake caliper and grabs the rotor slowing the wheel down to match the other. If both the ABS light and traction control light come on it's most likely a shared component like a rear wheel speed sensor. Because its intermittent it leads me to think its a ground wire or a dirty rear wheel spped sensor.