Hydraulic Brake Boost C123EF0 Error Code on Audi A4 (8W/B9) causes Red Warning Light

Bosch 9.0 ESP - ABS-units
Bosch 9.0 ESP


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What does this Hydraulic Brake Boost error code mean?

C123EF0 is an error code that is becoming more and more common on the Audi A4 8W / B9 that went into production from 2015. The error code means there is a problem with the Hydraulic Brake Boost and will cause a red warning light with an exclamation mark to be triggered (handbrake light).

Causes of Error Code C123EF0

The fault starts as a fault in the brake booster vacuum sensor. Once this fault is registered, the ABS system uses an accumulated pressure source to operate/increase the braking assistance. Once the accumulated pressure is released (brake pedal activated several times), the ABS system is unable to restore the system pressure. In short: in this case, the Bosch 9 ABS used in this car is the culprit causing the malfunction.

Important addition: Due to this fault, the brake pedal can lose its entire function if the fault is not fixed!

Our advice in case of this error code on the Audi A4

The Bosch 9 builds on the success of the Bosch 8: the software has a modular structure and different variants are available for different applications. Bosch claims that generation 9 has become 30% lighter, which means this ABS unit weighs only 1.1 kg. And despite this lower weight, this does not mean that the functionalities have been sacrificed. On the contrary. After extensive investigations by our R&D department in which the entire component has been reverse engineered, we can now also offer a remanufacture solution for this problem. That means no hassle with reprogramming, but above all a considerable saving. A new Bosch 9 with ESP functionality will set you back more than £2000! By opting for remanufacturing, you only pay a fraction of that amount.

So if you have an Audi A4 in your workshop with the error code C123EF0, take a look at our product page, where you can also register your remanufacture order. If you have any questions, our technical advisors will be happy to help you.