Repair ABS pump Audi C123EF0

What does this C123EF0 Hydraulic Brake Booster error code mean?

C123EF0 is an error code that is increasingly occurring in Audis that have been in production since 2015. The error code means that 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). After connecting the diagnostic computer, you can read the following error codes: C123EF0 - Hydraulic Brake Booster Limit Value Reached or 15381504 - Hydraulic Brake Boost.

Causes of error code C123EF0

The C123EFO fault starts as a fault in the vacuum sensor of the brake booster. Once this fault is registered, the ABS system uses an accumulated pressure source to operate/increase the brake assistance. Once the accumulated pressure drops (brake pedal activated several times), the ABS system can no longer restore the system pressure. In short: in this case, the Bosch 9 ESP ABS used in this car is the culprit causing the fault.

Important addition:

Due to this fault, the brake pedal can lose its complete function if it is driven for too long!

We recommend that after repairing ABS pumps with fault C123EF0 always the servo pressure sensor to replace.