P0573 Mercedes: Brake Switch “A” Circuit High
Complete Diagnosis, Repair Procedures & Cost Analysis – Everything You Need to Fix This Common Mercedes Issue
1.0 Understanding the P0573 Error Code
The P0573 – Brake Switch “A” Circuit High is an OBD-II diagnostic trouble code indicating a malfunction in your Mercedes-Benz’s brake switch circuit. This code specifically means the Engine Control Module (ECM) is detecting a higher than expected voltage signal from the primary brake switch circuit, even when the brake pedal is not depressed.
Description: Brake Switch “A” Circuit High
Severity: Medium – Affects drivability systems
DIY Difficulty: Intermediate (requires electrical testing knowledge)
Common Models: C-Class (W203, W204), E-Class (W211, W212), S-Class (W220, W221), ML/GL Classes
1.1 How the Brake Switch System Works
Modern Mercedes-Benz vehicles utilize a sophisticated dual-circuit brake switch system for redundancy and safety compliance. This system consists of:
- Brake Switch “A”: Primary signal circuit that communicates with the ECM, transmission control module, and cruise control system. This circuit sends a 5V or 12V reference signal to the ECM when the brake pedal is depressed.
- Brake Switch “B”: Secondary circuit that directly controls the brake lights and provides a redundant signal to additional control modules.
- System Logic: When the brake pedal is depressed, both switches change state simultaneously. The ECM expects to see corresponding signals from both circuits within a specific timeframe. A mismatch or unexpected signal triggers the P0573 code.
- Integration with Safety Systems: The brake switch signal is critical for proper operation of ESP (Electronic Stability Program), BAS (Brake Assist System), Distronic cruise control, and transmission shift interlock systems.
2.0 Symptoms of P0573 in Mercedes Vehicles
When the P0573 code is stored in your Mercedes’ ECM, you may experience one or more of the following symptoms, ranging from minor inconveniences to significant drivability issues:
- Illuminated Check Engine Light: The MIL (Malfunction Indicator Lamp) will remain lit until the issue is resolved. Some models may also store pending codes before triggering the full MIL.
- Cruise Control Inoperation: The most common symptom – cruise control will not engage or will immediately disengage when attempted. This is a safety feature to prevent unintended acceleration.
- Brake Light Issues: Brake lights may remain constantly illuminated (draining battery) or may not function at all (safety hazard). In some cases, they may work intermittently.
- Transmission Shifting Problems: Difficulty shifting out of Park or unexpected transmission behavior. The shift interlock system requires a proper brake signal to release.
- ESP/BAS Warning Lights: Electronic Stability Program and Brake Assist System warnings may appear on the dashboard, indicating reduced safety system functionality.
- Reduced Engine Performance: Some models may enter a limp mode with reduced power output to prevent potential safety issues.
- Start/Stop System Malfunction: In vehicles with start/stop technology, the system may not function properly or may display warning messages.
3.0 Common Causes of P0573 in Mercedes-Benz
Based on diagnostic data from Mercedes-Benz repair specialists and technical service bulletins, the P0573 code typically results from these issues, listed in order of probability:
- 3.1 Failed Brake Light Switch (Most Common – 65% of cases): The switch itself wears out internally, sending a constant high signal to the ECM. These switches contain delicate electrical contacts that degrade over time due to heat, vibration, and repeated use. Mercedes part numbers A0045458902, A0075459902 are commonly replaced.
- 3.2 Damaged Wiring Harness (20% of cases): Wires leading to the brake switch can chafe, break, or short circuit due to pedal movement. Common damage points include where the harness passes near the brake pedal mechanism, through firewall grommets, or near sharp metal edges. Look for rubbed insulation, exposed copper, or melted wires.
- 3.3 Corroded or Loose Connector (8% of cases): The electrical connector at the brake switch can develop corrosion or poor contact due to moisture ingress, temperature cycling, or vibration. The 4-pin connector (typically gray or black) may show green corrosion on pins or cracked housing.
- 3.4 Blown Fuse (3% of cases): A blown fuse in circuits related to brake lights or ECM power can cause irregular signals. Check fuses 6, 7, 38 (depending on model) in the front SAM (Signal Acquisition Module) unit. Use Mercedes fuse chart for specific model applications.
- 3.5 Faulty Steering Column Control Unit (2% of cases): On many Mercedes models, the brake switch signal passes through the SCCM (which also handles the turn signals, wipers, and steering angle sensor). A failure here can corrupt the signal. Diagnosing SCCM issues requires specialized STAR diagnostic tools.
- 3.6 ECM Malfunction (Rare – 2% of cases): Internal fault within the Engine Control Module itself (requires professional diagnosis). This is typically only considered after all other possibilities have been eliminated and may require ECU testing or replacement.
4.0 Diagnostic Procedure: Step-by-Step Guide
4.1 Preliminary Inspection
Tools Needed: Flashlight, basic hand tools (Torx set T15-T25), trim removal tools
- Verify the P0573 code with an OBD-II scanner. Check for additional codes that may indicate related issues (P0571, P0572, P0574).
- Check code freeze frame data to see under what driving conditions the code set (speed, engine temp, etc.).
- Locate the brake switch mounted on the brake pedal bracket under the dashboard. It’s typically a white or black plastic component with a 4-pin electrical connector and a plunger that contacts the brake pedal.
- Inspect the wiring harness for obvious damage, chafing, or melting. Pay special attention to where the harness bends with pedal movement.
- Check that the electrical connector is firmly seated. Unplug it and look for corroded or bent pins.
- Test brake light operation with pedal depressed and released. Have an assistant observe or use reflective surface.
- Check switch adjustment – there should be slight free play in the plunger when pedal is fully released.
4.2 Brake Switch Testing (Power Off)
Tools Needed: Digital multimeter, wiring diagram for your specific model, small pick set
- Disconnect the negative battery terminal for safety (prevents short circuits).
- Unplug the electrical connector from the brake switch.
- Set multimeter to resistance (Ohms Ω) mode with audible continuity if available.
- Identify pins for Brake Switch “A” circuit (consult wiring diagram – typically pins 1 & 2).
- Measure resistance across these pins with pedal released: Should show Infinite Resistance (OL – Open Loop).
- Measure resistance with pedal depressed: Should show Very Low Resistance (接近 0 Ohms, typically < 5Ω).
- Test Brake Switch “B” circuit (pins 3 & 4) using same procedure for comparison.
- If switch fails either test or shows intermittent readings, replacement is required.
- If switch tests good, the problem is elsewhere in the circuit.
4.3 Circuit Voltage Testing (Power On)
Tools Needed: Digital multimeter, back-probe pins or thin gauge wire, wiring diagram
- Reconnect the negative battery terminal and ensure connector is firmly attached to brake switch.
- Turn ignition to ON position (engine off) – do not start engine.
- Back-probe the wires for the “A” circuit using wiring diagram for pin identification.
- Check for reference voltage on pin 1 (typically 5V or 12V from ECM).
- Check signal voltage on pin 2 while operating brake pedal:
- Pedal released: Should read 0V or very low voltage (< 0.5V)
- Pedal depressed: Should read reference voltage (5V or 12V)
- If voltage doesn’t change appropriately, problem is in wiring, SCCM, or ECM.
- Check for voltage drop across the circuit by testing between ECM connector and switch connector.
- If all tests pass but code persists, suspect ECM programming issue or internal fault.
5.0 Repair Cost Analysis
Costs vary significantly based on the root cause, model year, and whether you choose DIY or professional repair. Labor rates for Mercedes specialists range from $120-$200/hour:
| Repair Scenario | Parts Cost | Labor Cost | Total Estimated | Time Required |
|---|---|---|---|---|
| DIY Brake Switch Replacement | $25 – $80 (OEM quality part) | $0 | $25 – $80 | 30-60 minutes |
| Professional Brake Switch Replacement | $40 – $100 (Mercedes OEM part) | $75 – $150 (0.5-1.0 hour) | $115 – $250 | 45-75 minutes |
| Wiring Harness Repair | $20 – $60 (wire, connectors, loom) | $150 – $400 (1-3 hours) | $170 – $460 | 1-3 hours |
| Steering Column Control Unit | $300 – $800 (new unit + coding) | $200 – $350 (1.5-2.0 hours) | $500 – $1,150 | 1.5-2.5 hours |
| ECM Diagnosis & Replacement | $800 – $1,500+ (new/reman ECU) | $300 – $500 (2-3 hours + programming) | $1,100 – $2,000+ | 2-4 hours |
| Dealer Diagnosis Only | $0 (diagnosis fee typically applied to repair) | $150 – $300 (1.0-1.5 hours) | $150 – $300 | 1-2 hours |
Cost-Saving Tip: Accurate diagnosis is key. Testing the brake switch first (as shown in section 4.2) can prevent unnecessary replacement of expensive components like the SCCM or ECM. Aftermarket brake switches can save money but ensure they include the necessary mounting hardware and adjustment features specific to your Mercedes model.
6.0 Frequently Asked Questions (FAQ)
While the vehicle may be drivable for short distances, it’s not recommended for extended periods or highway driving. The cruise control will be inoperative, and in some models, you may experience transmission shifting issues or reduced engine performance. Additionally, if the brake lights are malfunctioning (staying on or not working), it creates a significant safety hazard for you and other drivers. The illuminated warning lights may also prevent you from noticing other developing issues.
The brake switch is a mechanical component with electrical contacts that cycles thousands of times during normal driving. Over time, the internal contacts wear out or become contaminated with dust and debris. Some Mercedes models from the early 2000s (particularly W203 C-Class and W211 E-Class) had known issues with specific brake switch designs that were later revised. The location near the brake pedal also exposes the switch to heat from the footwell and potential moisture ingress, accelerating wear.
Follow the diagnostic procedure in section 4.0. If the brake switch tests correctly with a multimeter but the code persists, the issue is likely in the wiring harness, connector, or a control module. Look for damaged insulation, chafed wires, or corroded connector pins near the brake pedal assembly. A key indicator is if the problem is intermittent – wiring issues often come and go with vehicle movement or temperature changes, while a failed switch typically fails completely. Using a wiring diagram specific to your model is essential for accurate diagnosis.
While aftermarket switches are available at lower cost, Mercedes OEM parts are generally recommended for critical safety components. Aftermarket switches may have different durability standards and sometimes don’t include the necessary mounting hardware or adjustment features specific to your model. OEM switches typically last longer and provide more reliable operation. However, quality aftermarket brands like Bosch, Hella, or Febi Bilstein can be good alternatives if OEM is cost-prohibitive. Avoid no-name generic switches as they often fail prematurely.
If you’ve replaced the brake switch and the P0573 code returns, possible causes include: improper installation/adjustment of the new switch (there should be slight plunger free play when pedal is released), damaged wiring that wasn’t repaired, a faulty SCCM, or an issue with the ECM itself. The new switch could also be defective – even new parts can fail. Professional diagnosis with specialized Mercedes STAR diagnostic equipment may be required to pinpoint communication issues between modules or identify intermittent problems that basic scanning cannot detect.
Disclaimer: This guide is for informational purposes only. Automotive repair involves risks. If you are not comfortable performing these procedures, consult a qualified Mercedes-Benz specialist. Always prioritize safety when working on brake system components and disconnect the battery before beginning electrical work. Procedures may vary by model year and specific Mercedes-Benz model.
