If you drive an Audi from the 2010–2020 era—specifically an A4, A5, A6, Q5, or Q7—you may have encountered a terrifying red warning on your dash: "Brake system fault: stop carefully."
When you plug in a diagnostic tool, you’ll likely find Fault Code C123EF0 (Hydraulic Brake Booster – Limit Value Reached). Unlike most codes, this one is "permanent" and cannot be cleared by standard scanners, leading many owners to believe they need a $3,000 ABS module replacement.
The good news? There is a logical, much more affordable way to fix this if you understand how the system actually works.
1. The "Hidden" Logic Behind the Code
To understand C123EF0, you have to understand Hydraulic Boost Compensation (HBC).
Modern Audis use a vacuum-assisted brake booster to make the pedal easy to press. If the car detects that the vacuum is too low (meaning you'd lose power steering for your brakes), the ABS module activates its internal pump to provide "emergency" hydraulic pressure.
The Problem: The ABS pump is a backup, not a primary worker. Audi's software includes a safety counter. Every time the pump has to help because of low vacuum, the counter goes up. Once it hits a certain number, the computer assumes the vacuum system is permanently broken and "locks" the ABS module to prevent the pump from burning out.
This is why the code is permanent. The software has reached its limit and won't let the system run again until the counter is reset.
2. The Root Cause: Why did it lock?
The ABS module is rarely the part that actually failed; it is just the part that gave up. To fix this permanently, you must address the vacuum system first. The most common culprits are:
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Brake Booster Pressure Sensor: This is the #1 cause. If the sensor is faulty, it tells the computer there is "zero vacuum" even when the system is fine. The ABS pump works overtime to "fix" a problem that doesn't exist until the counter maxes out.
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Vacuum Leaks: Cracked rubber hoses or a leaking plastic check valve.
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Brake Booster Failure: An internal leak in the large "pancake" drum behind your master cylinder.
3. The Three-Step Fix
If you only replace the ABS module, the light will eventually come back because you haven't fixed the vacuum issue. Follow this sequence instead:
Step A: Replace the Sensor
Always start by replacing the Brake Booster Pressure Sensor. It is a relatively inexpensive part and is usually located on the vacuum line near the firewall. Even if it looks fine, if you have code C123EF0, this sensor is the primary suspect.
Step B: Unlock the ABS Module
Since the code is locked in the software, you have two modern paths that don't involve buying a new unit from the dealer:
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Software Reset: There are specialized technicians and software tools (using a DoIP/Ethernet cable) that can connect to your car's OBDII port and manually reset the HBC counter to zero. This is the fastest method as it requires no physical removal of parts.
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Module Remanufacturing: You can remove the electronic portion of the ABS unit (the black plastic box) and send it to a specialized repair lab. They will test the internal circuits, repair any damage, and reset the software before sending it back.
Step C: Clear Precursor Codes
Before you finish, ensure you clear any related vacuum codes, such as P050F (Brake Assist Vacuum Too Low). If the vacuum system is healthy and the counter is reset, the "Brake system fault" will vanish.