TT Brake Warning Lights Signal Booster Sensor Failure

I had a similar issue with my Audi A4 (2004) last year and being someone with some hands-on experience working on cars, I initially thought it was the brake booster too. The symptoms you describe match what I experienced, reduced braking power, warning lights, and that distinctive change in pedal feel. After taking it to a workshop, it turned out the brake system was incorrectly bled, causing air in the lines. The ABS system was triggering warnings because of inconsistent pressure readings. The fix was relatively straightforward, a proper brake system bleeding procedure. Total cost was 90 Euro for labor. The workshop found no issues with replacement parts or the brake booster pressure sensor, which saved considerable expense. Your TT might have the same root cause, but to provide more specific guidance, could you share: When did these symptoms first appear? Have you had any recent brake work done? Is the brake fluid level normal? Does the pedal feel spongy or just harder to press? These brake issues directly impact vehicle safety, so getting it checked professionally is crucial. A proper diagnosis will determine if its just air in the system or something more serious with the brake booster components.

Last service was done at 19584 KM, which is also my current mileage. Thanks for sharing your experience, it helps to compare symptoms. The issues started right after a heavy rainstorm and some puddle driving. The brake pedal feels more spongy than hard. Checked the brake fluid level yesterday and it appears normal, with no visible leaks underneath. No recent brake work has been done, the last brake service was just a routine check during annual maintenance about 8 months ago. The ABS warning appears out of nowhere during driving, not consistently at startup. The diagnostic tools at my local parts store showed error code P0571, but they werent sure about its reliability. What concerns me is that brake performance seems worse during cold starts.

Thanks for those details. Your situation sounds quite different from my A4 case, especially with the timing after the rainstorm. This additional info actually points away from my initial air-in-lines theory. The spongy brake pedal combined with the P0571 code and symptoms after water exposure suggests possible water intrusion affecting the brake system electrical components. When this happened on my A4, it turned out water had gotten into the brake light switch connector, causing similar warnings and affected brake booster operation. Using professional diagnostic tools revealed the brake booster wasnt receiving correct signals to modulate pressure. The fix involved: Drying and cleaning electrical connections, Replacing the brake light switch (120 Euro), Testing brake booster operation, Full system diagnostic reset Total repair cost was around 280 Euro including labor. Vehicle safety returned to normal after repairs, with proper brake pedal feel and no more ABS warnings. The cold start performance issues you mention are particularly telling, this often indicates the brake booster isnt building proper vacuum pressure initially. Definitely get this checked soon, as compromised brake performance in cold conditions poses significant safety risks. Have you noticed any correlation between wet weather and brake performance since that initial rainstorm?

Thanks for the detailed response and sharing that repair experience. Yes, I have noticed the braking issues get worse in damp conditions since that rainstorm. Given the correlation with weather and the diagnostic data you shared, I am now more convinced its likely an electrical issue affecting the brake booster system. The cold start brake performance problems and spongy pedal feel match what you described with your A4. The 280 Euro repair cost sounds reasonable considering the safety implications. I will have the brake light switch and connections checked first before replacing the brake booster pressure sensor. Your input about the professional diagnostic tools revealing signal issues has been really helpful. I will get it properly diagnosed rather than relying on the parts store scan. The unusual ABS warnings during driving rather than at startup does align with potential water damage to the electrical components. I will book it in for repair this week since brake performance being compromised in cold and wet conditions is too risky for daily driving. Thanks for helping narrow down the likely cause.