Audi ABS Fault Code C123EF0 (Hydraulic Brake Booster – Limit Value Reached)

If your Audi shows the ABS and traction control lights along with “Brake system fault — stop carefully”, and a scan reveals C123EF0 (15381504), the ABS pump/module has entered a protection lockout state. This fault occurs when a vacuum/booster issue triggers repeatedly. Once logged, it cannot be erased. 

Good news: we can test and rebuild your ABS—no coding required after refit, and no need to buy new. 

Book a repair: 

Symptoms owners notice 

• ABS and traction control (ESC/ASR) lights illuminated 
• Brake system warning lamp illuminated 
• Dash message: “Brake system fault — stop carefully” 
• Emergency braking assistance disabled; vehicle reverts to basic braking 
• C123EF0 stored in the ABS/ESP control unit 

What the code means 

C123EF0 - Hydraulic Brake Booster — Limit Value Reached.

On affected Bosch ESP/ABS units (commonly Bosch ESP9/9.0), the module monitors booster pressure/vacuum. If readings go out of range repeatedly, the module counts these events and eventually sets a non-volatile DTC that cannot be erased. 

Why the fault becomes “permanent” 

Before this fault appears, you may see “Brake servo: restricted. You can continue driving.” Although the car may remain driveable, an activation counter increments with each event/ignition cycle/brake press. When the limit is reached, the module logs C123EF0 permanently. 

To maintain braking assistance when vacuum is low, the ABS can activate Hydraulic Boost Compensation (HBC). HBC is a temporary strategy; it isn’t designed for continuous use. If a vacuum-related fault isn’t repaired and HBC is triggered too many times, the ABS deactivates electronic braking functions and stores C123EF0 permanently. This prevents the pump from overworking (and suffering internal wear from being overdriven). 

Common causes 

• Malfunction in the vacuum system for brake assistance 
• Vacuum leaks (lines/hoses, non-return/check valve, brake booster) 
• Defective or insufficient vacuum pump 
• Faulty brake booster or booster pressure/vacuum sensor 
• HBC repeatedly activated due to loss of vacuum supply 
• Prolonged driving with an unresolved vacuum system malfunction 

Related/precursor fault codes 

• P050F00 (261632) — Brake Booster Vacuum Too Low 
• C11EC04 (263936) — Pressure Sensor for Brake Booster 

Important: If the booster pressure sensor or a vacuum leak is not addressed, the ABS can re-trigger the same faults even after a repair. When your ABS pump is repaired, it is essential to replace the brake booster pressure/vacuum sensor at the same time and fix any leaks. 

What owners can do (step-by-step) 

1. Scan and confirm: Verify C123EF0 and note any precursor booster/vacuum sensor codes. 
2. Fix the cause: Replace the brake booster pressure/vacuum sensor and repair any vacuum leaks. 
3. Rebuild, don’t replace: Send your ABS unit for test and rebuild. We return your original unit, so no coding is required on refit. 
4. Refit & road test: Refit the repaired ABS, clear any secondary codes, and perform a brake function test. 
5. Prevent recurrence: Confirm the vacuum system holds within specification and that no booster-related codes remain. 

The solution (manufacturer guidance vs our service) 

Manufacturer guidance: If C123EF0 — Hydraulic brake servo limit reached is stored, the ABS unit must be replaced.  

Our service: Rather than replacing, we test and rebuild your original ABS unit and return it ready to refit with no coding required—saving costs into the £1000s. 

The vacuum system for braking assistance must also be fully inspected and repaired if you are buying new or having your pump repaired. Failure to correct the underlying vacuum malfunction will eventually cause the ABS unit to again exceed the HBC limit and shut down. 

Send your ABS unit to Remanx — we test, repair, and return your original unit. No coding required when you refit it, and turnaround is typically just a few working days. (Lifetime warranty available, provided all other faults on the vehicle are resolved—ongoing vacuum/booster faults can cause repeat ABS pump damage.) 

Vehicles commonly affected 

This failure affects a wide range of Audi—and some VW and Bentley—models equipped with Bosch ESP9-family ABS, see tabel below:

*Part numbers shown are examples of the control-unit. Suffix letters vary by calibration and build. Always confirm by scanning the vehicle (look for an ID like EV_ESP9BOSCH...) and by checking the module label.

Important: Platforms can change ABS suppliers mid-run. Treat “Fitted from” as a start guide, not a full model-year range. Use VIN/PR-codes and the actual module label to verify ESP 9.x and part number before ordering service.

Audi/VAG part numbers we see with C123EF0 

The following VAG part numbers are frequently associated with this failure. If yours matches one of these, we can test and rebuild it. (If you don’t see your number, there’s still a strong chance we can repair it—please contact us.) 

• 4K0 907 379: P, T (Audi A6/A7)
• 4M0 907 379: K, N, Q, R (Audi Q7/Q8)
• 4M6 907 379 AF (VW Touareg)
• 8W0 907 379: E, F, G, J, K, Q (Audi A4/A5/S4/S5)
• 80A 907 379 E (Audi Q5/SQ5)

Why choose repair over replacement? 

• Keep your original coding — no post-install programming 
• Fast turnaround
• Considerably lower cost vs. replacement 
• Tested and warranted repair (subject to underlying faults being fixed)