Brake pad wear sensor

you need to replace the pad wear sensor when the pad wears down it wears the sensor and the sensor needs to be replace if you look at the sensor you can see the wire on the sensor wears threw

This is the brake wear indicator. When the brake pads get too thin, this sensor, which is mounted in the brake pad backing on the front left wheel and the rear right wheel, touches the rotor and wears through, grounding out the contacts and illuminating the light.
Once the light is lit, you will need to buy a new sensor as well as new brake pads. The sensor is not expensive.
First step is to examine the brake pads and determine if you need front pads or rear pads or both. Don't be surprised if you need new rotors as well as pads. Check for a significant lip at the outer edge of the rotor. If the pads have worn in the sides of the rotor more than about 1/16", you are due for rotors.
You cannot go wrong with BMW branded pads. They are safe and work great, but they are really dirty. If you want to try other brands, read the reviews. Some squeak and some fade.
I use Mexican made Brembo rotors. They work well but they rust quickly, so the brakes squeak a bit the first time you press them after washing the car or driving in the rain. More expensive rotors or BMW branded rotors don't rust so much, but they cost a lot more and don't necessarily last any longer.
After replacing the pads, turn the key to the On position and leave it on for about 90 seconds before starting the car to reset the brake pad warning light.

Did you replace the wear sensors. If the sensors were activated(the light went on) before you changed the pads then the sensor must be replaced

Did you change the sensor? if you did, do a battery reset (disconnect for 10 to 20 min.).

Obviously the wear sensor has made contact with the rotor and is already damaged. You're not even suppose to resuse the wear pad sensor if its worn or damaged anyways.The way how the sensor works is that it must make contact to the rotor to complete the wear pad light indicator circuit. There is no mentioning that he replaced the the wear pad sensor with a new one.

But if one did replaced the wear pad sensor and the wear pad indicator is lit up, the way to reset it is to leave the key on in ignition switch #2 or #1 for about 30sec. turn it off and its reset.

Its possible that you forgot to replace the brake light sensors. They are about 15 bucks, and have to be replaced in order for that light to turn off. You can either get them from the dealer where they'll likely charge you 3 times as much, or you can go to http://www.turnermotorsport.com and get them there.

You have to reset the light by leaving the ignition switch on position #2 for about 30 sec. turn off the ignition and start the engine up and see if it goes out. I recommend buying a new wear pad sensor as well because now light will lit up when you're way below spec (less than 3mm)

Also, you could try with this procedure for one E39: Turning off the Check Brake Linings warning on the Instrument Cluster...
- so you've just changed the brake pads on your E39, along with the wear sensors. The old sensor(s) needed replacement cause you waited a bit too long before changing the pads, and wore through the sensor(s), turning on the BRAKE light on the dash. But wait, the light is still on? But there's a new sensor installed... what gives?!
- there is a procedure required in order to turn off the light/warning. With the ignition off, step on and hold the brake pedal. Now, while holding the brake pedal down, turn on the ignition switch to position 2 (just before cranking). Hold the brake pedal until the BRAKE light is off. Tada, the warning is gone and reset.

NOTE: This procedure worked on a 2001 E46 325i. It took about 15-20 seconds, but it cleared out w/o having to unhook the battery.

Other details find in the Owners Manual 2006-BMW-Z4m-Roadster-Owner-Manual

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Brake wear indicator and low brake fluid warning indication are two different circuits.

They are not related.

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Easy route to check brake pads is the measure the gab between the rotor and the metal portion of the brake pad.

If it's low then replace it.
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The right way is to pull the tires and measure the thickness of the remaining brake pad.

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Most shop give free brake inspection with oil change,

Check it out before the pad cuts into the rotor and it will DOUBLE the cost of your brake job.

As the brake pads wear, the brake pad wear sensor contained in the brake pad contacts the brake rotor. This causes the sensor to fail. When a pad wear sensor fails it opens the ground circuit of the pad wear warning light system causing the pad wear warning light to illuminate. Perform normal brake service to correct this condition and be sure to replace the brake pads with pads containing wear sensors. Once the brake pads and wear sensors are replaced, reset the brake pad warning system by turning the ignition switch to the "on" position. As long as continuity to ground exists through all the brake pad wear sensors the system should reset and the pad warning light in the dash will go out. If the pad warning light does not go out, a check of the pad warning circuit for an open circuit condition or poor ground must be performed.

i have replaced both front/ rear pads, and no matter what the dash light will not go off. I have disconnected both front/rear sensors, and light is still on , any suggestions?

no pad warning light on rears

Yes the parking brake uses a small set of brake shoes that act on a drum built into the rear rotor. There is no wear sensors on the parking brakes. There are however wear sensors on the front and rear brake pads. If your brake pad warnig light is on I would have the service brake pads inspected.

The brakes. It goes off because your brake fluid has gone below the minimum level, and sets the sensor off. Before you go adding brake fluid, what it REALLY means is that your brake pads are so worn down that the brake pistons in your calipers are extended all the way in order to make contact with the rotors. Therefore, the amount of fluid in your reservoir is low. Replace your brake pads, and see if you also need your rotors shaved/replaced because you let them go so far. Personal note: On older cars, the brake fluid is supposed to be replaced every 3 years. See if it needs a flush. If you can, drain out all the old DOT 3 brake fluid and replace it with synthetic DOT 3 or the newer DOT 4.