[PhaseP]

Recently replaced front pads and rotors. Got curious about the sensor workings and internal, so did some grinding with the one I removed from the car. The way it works is, it is a two stage sensor. Two wires come out of the sensor. When it is new, the wires are connected to each other at the tip of the sensor. As pad starts to wear, the sensor comes in contact with the rotor and it also starts to be eaten by the rotor, like a metal file removing material from the sensor as it turns. First this causes the connection at the tip of the sensor to break away, so the two wires are no longer directly connected. This is the first stage. At this stage there is a 470 ohm resistor inside the sensor that connects the two wires together. As the sensor is eaten more by the rotor with the pad thickness going lower and lower, at one point this 470 ohm connection is also broken, which leaves no connection between the two wires. At this point you get the full red light to replace the pads. Up to this point you may get replace pads warning based on computer estimate. It appears computer makes its estimate from mileage it took a new sensor to break the first stage.

If you had the second stage tripped, until you replace the sensor you will keep getting the error light.

But if you had only the 1st stage tripped, you will not get the error light if you don't replace the sensor UNTIL you attempt to reset the computer for brake service. Since you replaced new pads, you would like to update the interval for brake pad change warning. When you attempt this, car looks for a new sensor, which should have the wires connected directly with no resistance. It finds 470 ohms instead, and so immediately lights up the red break light and red car on lifts needs service image with -9000 miles service over due.

Good thing if you get new sensor, you can reset this the same way as you reset any service interval. Look up for E90 service reset DIYs.

If you just cut the wires and short circuit them, i.e. connect the wires to each other, computer takes this as a new sensor, which works also, but you won't be reminded when your brakes are down to metal, unless you check yourself manually.
On more regular cars, there is a metal tip attached to one brake pad, the comes in contact with the rotor when pad thickness is low, and makes a nasty sound when brakes are applied, which is a warning to replace the pads. Same idea, sensors are higher tech.

Here are pictures. The first one shows the 1st stage tripped. Since there is brake dirt on it not easy to see, but there is a short copper dash and a long copper dash exposed on the sensor face. When new these are connected and not exposed yet, covered with plastic.
The second picture shows the 2n stage tripped. I filed down the sensor face manually to get this. the long copper dash became two short dashes now, they got disconnected.
The third picture shows a cut away I got by grinding down with dremel. You can see the copper traces that go up to the sensor face. The silverish small part that is in the bend section is the 470 ohm resistor.

This page gives a schematic of the sensor
https://wiki.bentleypublishers.com/d...W+5+Series+E60