A4 Brake Booster Sensor Issue Causes Safety Concerns

Having some experience working on VAG cars, I encountered a similar issue on my 2003 A4 2.0T last year. The brake pedal behavior and error messages match what you described. Initially thought it was the ABS system due to the soft pedal feel, but after taking it to my regular workshop, diagnostic tools revealed the brake system was improperly bled during previous maintenance. This caused air in the lines, leading to reduced brake performance and triggering the vehicle safety warnings. The fix involved a complete professional brake system bleeding procedure using specialized diagnostic tools to ensure all air was removed from the system. Total cost was 90 Euro including labor. The brake pedal firmness returned to normal and all warning lights cleared. Before jumping to sensor replacement, having a proper diagnosis is crucial. Could you share: Current mileage, When was the last brake service performed, Are you experiencing any other symptoms, Which specific error codes are stored, Does the pedal feel worse when engine is cold or warm This information would help determine if your issue matches what I experienced or if the pressure sensor might actually be the culprit.

Latest service was done at 121056 KM which included brake fluid change, but the problem started shortly after. The diagnostic tool showed error code P0504. While the brake fluid change temporarily helped, the brake pedal gradually became spongy again over two weeks. The ABS system warning light started flashing intermittently during this period. The pressure sensor theory makes sense based on the symptoms described, but I want to be certain before spending money on replacement parts. The brake pedal feels consistently soft regardless of engine temperature. Looking to gather more insights before booking another workshop visit.

Thanks for providing those additional details. My 2003 A4 showed identical symptoms after a brake fluid service, including the same error code P0504. While air in the hydraulic brakes initially seemed the likely cause in my case, it turned out the brake booster pressure sensor had indeed failed. The key indicator was the consistent pedal softness regardless of engine temperature, exactly like your situation. After replacing the faulty sensor (120 Euro for genuine replacement parts plus labor), the brake system returned to normal operation and the error codes cleared permanently. The intermittent ABS warning is significant here, this happened on my vehicle too when the sensor was sending irregular signals to the brake control module. A proper diagnostic scan will typically show erratic pressure readings from the faulty sensor. Since you mentioned the symptoms appeared shortly after service, consider having the workshop verify their brake fluid change procedure. However, based on the matching error code and behavior pattern, I strongly suspect you are dealing with the same brake booster sensor failure I experienced. Would recommend getting a complete system diagnosis using professional diagnostic tools before proceeding with repairs, but be prepared that sensor replacement may be necessary to resolve this safety-critical issue.

Thanks for the input. Given the matching symptoms and especially the P0504 code, I am now convinced the brake booster pressure sensor is likely the root cause. The fact that another A4 owner experienced identical issues, including the ABS system warnings and consistent pedal softness, helps confirm my initial suspicion. I will schedule an appointment to have proper diagnostic testing done using professional diagnostic tools to verify the sensor readings. Based on the shared experience, I should budget around 120 Euro for replacement parts if the sensor needs replacing. Having a clear diagnosis and repair path is reassuring since brake system issues cannot be ignored. Will make sure the workshop also verifies their previous brake fluid service was done correctly, but the evidence points strongly toward sensor failure rather than air in the lines. Really appreciate the detailed technical insights, helped me understand what I am dealing with before spending money on repairs.