S4 Brake Booster Sensor Failure Symptoms & Fixes

As someone with some hands-on experience working on brakes, I encountered a similar issue with my 2011 S5. The symptoms you describe match exactly what I experienced, decreased brake performance and a stubborn brake pedal. Initially, I also thought it was the pressure sensor, but my workshop discovered the hydraulic brakes needed proper bleeding. The brake fluid had air bubbles due to incorrect bleeding during routine maintenance. This caused inconsistent brake pedal feel and triggered the warning light. The fix was straightforward but required professional equipment. The workshop performed a complete brake system bleed using their diagnostic tools to ensure all air was removed. Total cost was 90 Euro and this resolved both the pedal feel and warning light issues. Before jumping to replacement parts, you might want to have the brake system checked for proper bleeding first. This could save you from unnecessary sensor replacement. To help diagnose your specific case, could you share: Current mileage, When was your last brake fluid change, Any recent brake work performed, Does the pedal feel change when engine is cold vs warm, Are there any changes in vehicle safety systems like ABS or stability control These details would help determine if your issue matches what I experienced or if you might be dealing with a different brake-related problem.

Last service completed at 154724 KM, and after reading all replies, I can confirm it was indeed the brake booster pressure sensor. Had it replaced along with a complete brake fluid flush. The vehicle safety systems are now working perfectly, and the brake pedal feel has returned to normal. The total repair cost was 280 Euro including parts and labor. This fix resolved all warning messages in the instrument cluster. The hydraulic brakes are now operating as they should, with consistent pedal pressure and proper system feedback.

Thanks for the update on your S4 repair. Good to hear the brake booster pressure sensor replacement fixed your issues. This confirms my initial suspicion even though my S5 problem was different. The consistent brake pedal feel and restored ABS system functionality are key indicators the diagnosis was correct. The 280 Euro repair cost seems reasonable for the sensor replacement and brake fluid service. For reference, pressure sensor failures often show these symptoms: Inconsistent brake pedal resistance, Warning lights in cluster, Compromised brake performance, ABS system warnings Regular brake fluid changes and monitoring pedal feel can help catch these issues early. The hydraulic brakes rely heavily on proper sensor readings, so maintaining clean fluid is essential for optimal performance. Would you mind sharing if you noticed any early warning signs before the complete sensor failure? This information could help others spot similar issues sooner.

Your feedback on the S4 repair was very helpful. I noticed some minor changes in brake pedal consistency about 2 months before the complete failure, but initially dismissed them as normal variation. The pressure sensor degraded gradually, which is why I first noticed slightly inconsistent pedal feel during normal braking. The diagnostic tools at the shop quickly identified the faulty brake booster sensor. Looking back, I should have acted when I first felt changes in the brake system response. The repair cost of 280 Euro including the brake fluid flush was worth every cent for the peace of mind and restored safety systems. For anyone facing similar symptoms, I recommend getting it checked immediately when brake pedal feel becomes inconsistent. Early intervention can prevent more serious brake system issues and ensure the ABS system stays fully functional. The repair process was straightforward once properly diagnosed, and the hydraulic brakes now work perfectly.