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Reduced braking power, start engine warning on Mercedes-Benz vehicles usually comes up because the auxiliary battery is too weak or completely discharged.

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  • A red warning light on the vehicle's cluster  "Brake Reduced Brake Effect -start engine."

Why would the SBC Braking System throw a "Brake Reduced Brake Effect" alert?

SBC would throw the above error when:

  • The secondary battery is low
  • The brake light switch is defective
  • SBC pump is defective
  • Brake pads are worn

How to fix a "Brake Reduced Brake Effect" alert due to a battery issue?

To fix the BRBE alert due to a battery issue,  start the engine and let the engine run for a while to charge the battery.

If the battery is too weak, replace the battery and start the engine.

Brake reduced brake effect.Battery issue on a Mercedes Benz.

Usually, this will fix the "Brake Reduced Brake Effect" alert caused by low battery output.

What does this message mean? 

Mercedes Benz Brake Reduced Brake Effect

SBC Braking System is an electronically controlled brake system that Mercedes-Benz has fitted to its passenger car models. The system monitors the car and driver's actions through different sensors and sends appropriate braking power with just a small effort from the driver.  

SBC has various features, including distributing brake force to different wheels during cornering to ensure greater stability in the vehicle and maximum stopping power; the SBC system even dries the brake discs if a film of water forms on them.

The “Reduced braking power, start engine” warning will only appear in those Mercedes-Benz vehicles that have a Sensotronic Brake Control, or SBC.

This is a unique brake system that Mercedes has developed, which makes them quite different from most other vehicles.

A traditional brake setup has a mechanical linkage that connects the pedal to the brake booster and other corresponding hardware.

Unlike that, SBC has a module with an electric pump, a reservoir that holds pressurized brake fluid, and a series of control valves. When the driver presses the brake, the module distributes the fluid toward each wheel to optimize their braking performance.

Not only does this shorten the overall stopping distance, but it also makes it more stable during sudden and harsh brakings. 

Why does it come up? 

Mercedes reduced brake effect start engine

SBC may appear as a revolutionary system that improves vehicle safety, although real-life situations proved quite troublesome. There are several reasons for this, with warning messages that keep appearing without any apparent reasons.

Specific errors such as Reduced braking power, start engine do not impact the braking systems; many car owners find them annoying and confusing.

This is because of the safety-critical nature of the braking system, which leaves no room for failure of its components. The Sensotronic Brake Control relies heavily on sensors and electrical parts, whose irregularities can seriously offset the braking performance.

When any SBC components malfunction, the vehicle will warn the driver about potential hazards with several brake-related messages.

In some cases, it may also temporarily disable features such as interior lights, sunroof, or even the radio. The purpose of this feature is to draw drivers’ attention to the ongoing problem, although many of them find it frustrating. 

Common causes 

weak battery causes brake reduced brake effect
  • Consumer battery - The heart of the SBC system is its brake module, which has its battery as a reliable power source. This ensures a steady voltage for a reliable operation and allows the system to pressurize before the engine starts. If this consumer battery goes flat, it will trigger all sorts of warnings and may even cause brake issues. One of them is the “Reduced braking power, start engine” message, which will appear immediately after turning on the ignition. Replacing the battery is a simple job, although finding its location may be tricky in some models. 
  • Defective Brake Light Switch - A faulty brake light switch is one of the most common issues that cause this and many other similar warnings. Mounted at the top of the brake pedal, this sensor tells the vehicle when pressing the brakes. This is a simple part, although its internal components may eventually wear down and cause it to malfunction. When that happens, it will feed incorrect data to the SBC module, affecting the performance of those systems that use it. Replacing this inexpensive component is a simple job that requires only essential tools and some effort. 
  • SBC module, which has an electric pump with a hydraulic reservoir and a series of control valves, is another weak spot. Common fault code is C249F error - Operating time of Component of A7/3 is exceeded. In most cases, the electric motor will fail to operate as its internal components wear out. This is a severe issue, as it may prevent the pump from creating sufficient fluid pressure and obstruct brake operation. Being a widespread problem, there are many rebuild kits available on the market. Still, this is not an easy job, as it requires extensive experience and special tools. 

How to diagnose and solve the problem 

fix mercedes reduced brake start engine

If the “Reduced braking power, start engine” pops up immediately after turning the key, a weak consumer battery is a likely cause.

Car owners should avoid jumping to conclusions when diagnosing any SBC related issues due to their complexity. Instead, it is always best to use a scan tool capable of accessing its control module.

This gives a clear insight into stored error codes and running parameters, making the troubleshooting procedure more accurate. 

Additionally, owners should always contact an authorized dealer when there is an issue with the SBC system in their cars. Mercedes-Benz offers an extended warranty for vehicles with the SBC system to ease the criticisms they've been facing for years.

As a part of it, dealers will replace a faulty SBC module with a new one, totally free of charge. This warranty originally covered vehicles up to 10 years old, only to be extended to an unprecedented 25 years.

However, it is worth noting that this warranty only affects registered and operated vehicles in the US. 


As we explained here, a “Reduced braking power, start engine” warning is usually a telltale sign of an SBC system issue. In most cases, this will boil down to a weak consumer battery, which provides electricity for its main module.

Other possible causes include a faulty brake pedal switch and a malfunctioning SBC module. Whatever may be the cause, we suggest using an appropriate diagnostic tool to track it down.

If one of the SBC components is faulty, contact Mercedes-Benz authorized dealers, as they offer an extended warranty. This means they may cover this repair even if the vehicle warranty has expired. 

If the Mercedes-Benz dealership didn't fix your vehicle under warranty, the next step would be to read the fault codes yourself. 

Reduced Braking Power Start Engine Warning Mercedes-Benz Reduced braking power, start engine warning on Mercedes-Benz vehicles usually comes up because the auxiliary battery is too weak or completely discharged.