Audi ABS pump repair C123EF0

What does this C123EF0 Hydraulic Brake Booster fault code mean?

C123EF0 is a fault code that is increasingly common in Audi models produced since 2015. The fault code indicates a problem with the Hydraulic Brake Boost and will cause a red warning light with an exclamation mark (handbrake light) to be triggered. After connecting a diagnostic computer, you may read out the following fault codes: C123EF0 - Hydraulic Brake Booster Limit Value Reached or 15381504 - Hydraulic Brake Boost.

Causes of fault code C123EF0

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

Important addition:

Most common causes of fault code C123E F0

● Defective brake booster pressure sensor

The Brake Booster Pressure Sensor is located on the vacuum hose, directly at the brake booster.

• This sensor must be replaced by the latest, improved version.
• The first versions had a factory defect, which often leads to this fault.

● Problem with the vacuum pump

The vacuum pump is the second most common cause of this fault.

• The vacuum pump is integrated into the oil pump.
• This pump also had production faults in earlier versions.
• Replacement by an updated model is necessary.

● Leakage in vacuum hoses

• Wear or cracks in the hoses
• Damage by animals (e.g., martens)
• Poor or loose connections

● Leaking brake booster

• Internal leakage of the brake servo
• Insufficient vacuum buildup for proper brake assist

Important installation instruction

Only after all vacuum problems have been fully resolved can the repaired ABS/ESP pump be safely installed.