C123E

C123E - Hydraulic Brake Booster: Limit Value Reached

Possible Symptoms

• ABS warning lamp ON
• ESC (ESP/ASR) warning lamp ON
• Brake system warning lamp ON

Possible Causes

• Malfunction in the vacuum system for brake assistance
• Vacuum leaks (lines, check valve, brake booster)
• Defective or insufficient vacuum pump
• Faulty brake booster or vacuum sensor
• Hydraulic Boost Compensation (HBC) activated repeatedly due to loss of vacuum supply
• Prolonged driving with unresolved vacuum system malfunction

Possible Solution

• If DTC C123EF0 Hydraulic brake servo limit reached is stored, the manufacture says the ABS unit must be replaced.
• At the same time, the vacuum system for the braking assistance must be fully inspected and permanently repaired, even if the malfunction appears intermittent or cannot be reproduced at the time of service.
• Failure to correct the underlying vacuum malfunction will eventually cause the new ABS unit to again exceed the HBC limit and shut down.

Special Notes

• If there is a malfunction in the vacuum system for the braking assistance, the ABS control unit activates Hydraulic Boost Compensation (HBC) to maintain braking assistance.

• However, only a limited number of HBC activations are permitted. Once this limit is reached, the ABS control unit permanently deactivates braking electronics and logs the event: C123EF0 Hydraulic brake servo limit reached.

• At that point, the ABS unit must be replaced.
• This is not a random failure of the ABS unit, but rather a safeguard triggered because a vacuum-related malfunction was not repaired in a timely manner.
• See the Bentley Motorcars Technical Service Bulletin 2053923 – Although this bulletin is written for Bentley vehicles, the information and condition described may also apply to other Volkswagen Group brands