[Efthreeoh]

You've not provided enough information to really help, but I'm quite familiar with the modern CBS-era brake system, so I'll take a shot. I've owned BMWs for 30 years and do all my own maintenance and repairs. I have an E90, which has a similar CBS system as your E89 Z4 does. BMW's have had brakepad wear indicators since the mid 1980's. The CBS-era brakepad wear indicators are dual stage so they can estimate pad life remaining and show it in the CBS maintenance menu.

So first off, you don't say how many miles are on your car, or if you know the brakes are the original brakes or if they have been replaced once already. You also stated your "mechanic" said the brake system was fine. I have to ask if your mechanic is a trained BMW mechanic. A lot of non-BMW mechanics do not understand BMW brake systems. BMW leaves quite a bit of pad material (3 - 5 MM of material) on the pads when the wear sensors indicate the pads need replacement. Mechanics with little understanding of BMW brake systems will look at the pads (still in the calipers) and think the pads are not at the wear limit. The CBS will report the estimated miles left for the brake pads for each axle separately, so the front pads could need replacement and the rear pads not, or vice versa. The sensors are mechanical wear limit indicators that have a loops of electrical wire embedded in the sensor plastic body that wears down over time until the wires break, which then triggers the light.

If all other parts of the brake system, including the fluid level, are up to spec, once the wear notification comes on it means the pads on either the front or rear axles need replacement (or possibly both axles). Clearing/resetting the notification in the CBS is not same thing as resetting the pad wear notification. The CBS just estimates remaining pad life and is not an indication the pads need replacement. The red "brake" notification means the pad wear sensors have worn to the pad limit and the pads need replacement. The only way to clear the red "Brake" pad wear notification is to replace the pads and wear sensors. If your car is still on the original factory brakes, and has over 50,000 miles on the original brakes, and no one has messed with the brake system, then I'm pretty sure at least the pads on one of the axles (front or rear) need replacement. Now that you have reset the CBS, you've removed the ability to figure out which axle needs replacement. A visual inspection of the brakes by a trained BMW mechanic familiar with the BMW CBS system will be able to measure the pads and figure out which pads need replacement.

That said, now that you'll need a mechanic to review the brakes because the brakepad wear light is on, you might as well just get all the brakes and sensors replaced and start over. The sensors are consumables and wear along with the brake pads, so they need to be replaced as a set with the new pads. If one of the axles needs new pads then the other axle is close behind in needing pads as well. Modern BMWs have a balanced braking system, which under normal driving conditions generally wear all four brakes quite evenly. And lastly, if you get the brakes replaced, have the shop replace both the pads AND rotors. Usually the rotors wear below their minimum spec before the new (second set) of pads wear out.

Good luck with it.