S3 Brake Booster Sensor Issues & Poor Braking Performance

Had this exact issue on my 2009 A3 2.0T last year. Have some experience working on VAG cars, mostly basic maintenance and repairs. The symptoms match what I experienced, sluggish brake performance and error messages. Initially thought it was a failed brake booster or pressure sensor, but the real problem turned out to be improper brake system bleeding after a routine brake fluid change. Took it to my regular workshop where they performed a complete diagnostic check of the vehicle safety systems. They found trapped air in the brake lines which was causing inconsistent pressure readings and throwing off the sensors. A proper brake system bleed solved everything, total cost was 90 Euro including labor and fresh brake fluid. The speedometer error is often linked to the brake system because modern cars integrate multiple safety systems. Before replacing expensive parts, having a proper system bleed and pressure sensor calibration is worth trying. Could you share: Current brake fluid age, When brakes were last serviced, If any recent brake work was done, Whether the brake pedal feels spongy or firm This would help determine if your issue matches what I experienced or points to a different problem.

Thanks for the responses so far. Just had the 155000 KM service completed last month where they changed oil and filters. To answer your questions: brake fluid is 2 years old, brakes were serviced during the last major service 6 months ago. No recent brake work besides regular maintenance. The brake pedal feels increasingly spongy when pressing multiple times. After checking the fault codes again with my diagnostic tools, the hydraulic brakes seem to be losing pressure somewhere in the system. Think I will have the workshop do a thorough check of the brake booster and related components since the symptoms persist even after bleeding the system properly. The speedometer error appears out of nowhere, which makes me suspect an electrical issue with one of the vehicle safety sensors rather than just trapped air. Would appreciate any other suggestions before booking it in.

Thanks for the additional details. The symptoms you describe, especially the spongy brake pedal getting worse with multiple presses, match my experience before getting mine fixed. However, your case sounds more complex since bleeding didnt resolve it. In my A3, after the initial brake fluid bleed failed to fix the issue, we discovered a hairline crack in the brake booster housing causing a slow pressure leak. The unusual speedometer errors were triggered by inconsistent pressure readings from this leak. The diagnostic tools showed similar fault codes related to the hydraulic brake system. The repair required: Complete brake booster replacement (590 Euro), New pressure sensor (120 Euro), System bleeding and calibration (90 Euro) Total repair time was about 4 hours. The brake performance returned to normal and all error messages cleared. Given your mileage and symptoms, I would suspect either the brake booster itself or a failing pressure sensor causing erratic readings. Worth having the workshop pressure test the entire brake system to locate any potential leaks before replacing parts. Make sure they check all brake lines and connections too, as high mileage can cause subtle deterioration that affects brake performance.

Thanks for sharing your experience. Your case sounds very similar to mine, particularly regarding the pressure leak symptoms. Will definitely have them check the brake booster housing for cracks first. The ABS system throwing unusual errors combined with the spongy brake pedal does point toward a pressure-related issue. Since the brake performance keeps deteriorating even after bleeding, I agree its likely more than just trapped air. Given the age and mileage of my car, investing in replacement parts like a new brake booster and pressure sensor might be the best long-term solution for maintaining proper vehicle safety. The repair costs you mentioned seem reasonable considering the components involved. Will book it in next week for a full hydraulic brake system inspection and pressure testing. Better to address this properly now rather than risk further brake performance issues. Will update once I know the exact problem and repair outcome.