A vehicle purchased from an auction presented with a faulty dashboard insert and constantly illuminated brake lights with the ignition on. Troubleshooting this issue revealed a severed wire between the brake switch and the Body Control Module (BCM). Despite repairing the wire, the brake lights remained active. Further investigation with VCDS diagnostics uncovered a peculiar finding: a fault frequency of one for the brake light switch error. This article delves into this specific scenario and explores potential causes and solutions.
Troubleshooting Brake Light Issues with VCDS: A Case Study
Initial inspection of the vehicle revealed missing and incorrectly rated fuses, which were replaced according to the wiring diagram. A cut wire connecting the brake switch to the BCM was discovered and repaired. However, the brake lights persisted in staying on.
Testing the brake light switch itself confirmed its functionality: it registered battery positive when the brake pedal was pressed and no voltage when released. This indicated the problem lay elsewhere.
Figure 1: Initial VCDS scan revealing multiple faults, including an implausible signal from the brake light switch.
A key discovery was made when removing fuse SB15, responsible for the CAN Gateway module. With this fuse removed, the brake lights functioned correctly: off with the ignition on and illuminating when the brake pedal was pressed. Reinserting the fuse immediately reactivated the constant brake light issue.
Figure 2: VCDS live data indicating the brake switch signal via CAN as the potential source of the problem.
Live data monitoring through VCDS pointed towards the “Brake switch via CAN” as the root cause. But pinpointing the exact origin of the problem required further analysis. The persistent illumination of the brake lights despite a functional switch suggested a communication error within the CAN bus system.
VCDS Fault Frequency: Decoding the “Always One” Phenomenon
Examining the fault codes revealed a consistent “Fault Frequency: 1” for the brake light switch error (00526 – Brake Light Switch (F), 008 – Implausible Signal). This indicates the fault was logged only once.
Figure 3: Close-up of the fault code 00526, showing a fault frequency of 1.
A fault frequency of one, especially when the problem is persistent, can be misleading. It suggests a sporadic error rather than a continuous issue. However, in this case, the problem was constant. This discrepancy hints at a potential issue with how the fault is being detected and logged, possibly due to a communication breakdown within the CAN bus system or a faulty module intermittently disrupting the signal.
Potential Causes and Further Diagnostic Steps
Several possibilities could explain this scenario:
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Faulty CAN Gateway Module: Given the brake lights’ correct operation with the SB15 fuse removed (disabling the CAN Gateway), this module is a prime suspect. A faulty gateway could be sending erroneous brake switch signals on the CAN bus, overriding the correct signal from the functional switch.
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Wiring Issue within the CAN Bus: Despite repairing the initial severed wire, other wiring problems within the CAN bus system could be causing communication errors. A thorough inspection of the CAN bus wiring, including checking for shorts or open circuits, is recommended.
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Faulty BCM: Although less likely, a malfunctioning BCM could misinterpret or fail to process the correct brake switch signal from the CAN bus.
Further diagnostic steps include:
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Checking for other related fault codes: Other modules connected to the CAN bus might have logged errors related to communication issues.
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Monitoring CAN bus traffic with VCDS: Observing the data flow on the CAN bus can help identify any unusual activity or signal inconsistencies.
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Testing the CAN Gateway module: Specialized tests can be performed to verify the functionality of the CAN Gateway module.
Conclusion: Addressing the Root Cause
The “Vcds Fault Frequency Is Always One” scenario in this brake light issue highlights the importance of looking beyond the immediate fault code information. While the frequency indicated a single occurrence, the persistent problem suggested a more complex underlying cause. By systematically investigating the system, focusing on the CAN bus communication and the modules involved (particularly the CAN Gateway), the root cause of the problem can be identified and resolved.
