03/09/2026
**Audi ABS Fault – C123EF0**
Got an Audi with an **ESP9 ABS control module** showing:
**C123EF0 – Hydraulic Brake Booster: Limit Value Reached?**
This fault can relate to an internal **pressure sensor issue** within the vehicle (External to the Module). After repeated key cycles, the system can lock the fault into the control module, leaving you with a very expensive replacement ABS unit.
We can repair the existing ESP9 ABS control module, saving your customer from the cost and hassle of sourcing and programming a brand-new unit.
Common across Audi models from approximately **2010–2020**, including the:
A4, A5, A6, A7, S4, S5, Q5 and Q7.
Don’t replace it; repair it.