P0557 on 2018-2021 Buick Enclave: Brake Booster Sensor Circuit Low Causes and Fixes

What's Unique About the 2018-2021 Buick ENCLAVE

The 2018-2021 Buick Enclave, part of the GM C1XX platform, has a documented history with this specific code. General Motors has issued multiple Technical Service Bulletins (TSBs) acknowledging that the original brake booster vacuum sensor is prone to failure. TSB 17-NA-314 identifies an internal 'circuit board fault' in the sensor. A later TSB, 20-NA-071, addresses potential EMI (electromagnetic interference) in the engine harness and poor terminal tension at the sensor connector as additional causes. This makes it a pattern failure for this vehicle and its platform mates (like the Chevy Traverse and GMC Acadia), with specific, updated parts and procedures available for the fix.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Service Engine Soon / Check Engine Light is on.
• "Service Power Brake Assist" message displayed on the driver information center (DIC).
• Brake pedal is harder to press than usual.
• Increased stopping distance.
• Brake shudder or unusual feel when braking.
• Possible hissing noise from the brake booster area if a vacuum leak is the cause.

Most Likely Causes

1. Faulty Power Brake Booster Vacuum Sensor / Pipe Assembly 🔴 High Probability This is a well-documented failure. TSB #17-NA-314 states the original sensor can fail due to an internal 'circuit board fault.' Another TSB notes the sensor can be affected by 'inconsistent in-rush current' during engine start/stop cycles, prompting GM to release a more robust replacement part.
   How to confirm: Using a scan tool, observe the brake booster pressure sensor data. If the reading is stuck at a low value (near 0 kPa or 0V), is erratic, or doesn't change when the engine is running and the brake is applied, the sensor assembly is likely faulty. Also, inspect the part for any visible damage.
   Typical fix: Replace the entire power brake booster vacuum pipe assembly, which includes the integrated sensor.
   Est. part cost: $70-$150
2. Damaged Wiring or Poor Connection 🟡 Medium Probability TSB #20-NA-071 specifically identifies 'poor terminal tension' at the sensor connector and 'potential for EMI in the engine harness' as causes for P0557 or P0558. This indicates the factory wiring is a known weak point.
   How to confirm: Visually inspect the wiring harness and connector going to the brake booster pressure sensor. Look for any signs of chafing, corrosion, burnt spots, or loose pins in the connector. Use a multimeter to check for 5V reference voltage and a good ground at the sensor connector with the key on, engine off.
   Typical fix: Repair the damaged section of the wiring harness or clean/replace the connector. TSB #20-NA-071 outlines a specific procedure to install a new external wiring harness (Part No. 84802275) to bypass the problematic section of the factory harness.
   Est. part cost: $5-$75 (depending on simple repair vs. harness replacement)
3. Vacuum Leak ⚪ Low Probability
   How to confirm: With the engine running, listen for hissing sounds around the brake booster and its vacuum lines. You can also spray a small amount of brake cleaner or starter fluid around suspected leak areas; a change in engine RPM will indicate a leak. However, for this specific code on this vehicle, the sensor itself or its wiring is a more likely cause than a leak.
   Typical fix: Replace the cracked or disconnected vacuum hose.
   Est. part cost: $10-$40

Rare But Worth Checking

• Failing Power Brake Booster: While the booster itself can fail, it's much less common than the sensor issue. A failing booster might cause a consistently hard pedal or hissing, but the P0557 code specifically points to the sensor circuit. This should only be considered after the sensor and wiring have been ruled out. A replacement can cost between $530 and $661.
• Faulty Powertrain Control Module (PCM): This is extremely rare. The PCM is the last component to suspect. All other possibilities, especially the sensor and wiring, must be exhaustively tested before considering PCM replacement.

Diagnosis Steps

1. Connect an OBD-II scanner and confirm P0557 is the active code. Note any other codes, especially P0558 or C025E.
2. Check for relevant Technical Service Bulletins (TSBs), specifically #17-NA-314 and #20-NA-071, which directly address this code on your vehicle.
3. Visually inspect the brake booster vacuum sensor and its integrated pipe/hose assembly. The sensor is located on the black plastic vacuum line running from the engine to the brake booster on the driver's side of the engine bay.
4. Inspect the sensor's electrical connector and wiring for any damage, corrosion, or loose connections. Per TSB #20-NA-071, poor terminal tension is a known issue.
5. With a scan tool, monitor the live data for the 'Brake Booster Pressure' parameter. The reading should change in response to engine vacuum and brake pedal application. If it's stuck low (near 0V) or reads erratically, the sensor or wiring is the primary suspect.
6. If a scan tool is unavailable, use a multimeter. Disconnect the sensor and turn the ignition to 'ON'. Check the harness connector for a 5V reference signal on one pin and a good ground on another. The expected voltage is typically between 4.9V and 5.1V.
7. If power and ground are good, reconnect the sensor and back-probe the signal wire. With the engine running, the voltage should vary with brake application. A stuck low voltage (under 0.5V) points to a bad sensor.
8. Given the high probability of sensor failure documented in TSBs, if wiring and connections are intact, the most logical step is to replace the power brake booster vacuum pipe/sensor assembly.

Parts You'll Likely Need

• Power Brake Booster Vacuum Pipe with Sensor (OEM #85572832 (supersedes 84396439, 84341899, 84298056)) — This is the specific part identified in GM service bulletins as the cause of P0557 due to internal sensor faults. It is an updated, more robust part designed to fix the issue. TSB 17-NA-314 originally cited older part numbers.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $70-$150
  Aftermarket price range: $60-$120
• Engine Wiring Harness Jumper (OEM #84802275) — Required if performing the repair outlined in TSB #20-NA-071, which involves installing a new external harness to bypass potential EMI or terminal tension issues in the original wiring.
  Trusted brands: GM Genuine Parts
  OEM price range: $40-$75
  Aftermarket price range: N/A

Related Codes That Often Appear With This One

• P0558 — P0558 is 'Brake Booster Pressure Sensor Circuit High'. GM TSB #20-NA-071 mentions that P0557 and P0558 can occur for the same root cause (wiring/EMI issues), indicating an erratic sensor signal rather than a consistent low or high voltage.
• C025E — This code is for 'Brake Booster Vacuum Sensor Signal Plausibility Failure'. TSB #17-NA-314 notes that P0557 and C025E can be set together, as they both point to the sensor providing incorrect vacuum values.

Technical Service Bulletins (TSBs) & Recalls

• 20-NA-071: Information on Service Engine Soon Lamp illuminated and/or Service Power Brake Assist message displayed with DTC P0557 or P0558. Correction involves replacing the hose assembly and installing a new external wiring harness.
• 17-NA-314: Correction for intermittent Service Power Brake Assist message, brake shudder, and DTC P0557 due to a faulty power brake booster vacuum sensor circuit board.

Platform-Specific Known Issues

• TSB #17NA314: Specifically addresses an 'Intermittent Service Power Brake Assist Message With a Brake Shudder and Possible DTC P0557'. It identifies the cause as the 'Power Brake Booster Vacuum Sensor providing intermittent incorrect vacuum values due to circuit board fault.' The fix is to replace the booster vacuum check valve and hose assembly.
• TSB #20NA071: Provides a more involved correction for 'Service Engine Soon Lamp illuminated and/or Service Power Brake Assist message displayed, with DTCs P0557 or P0558 Stored.' This TSB points to 'poor terminal tension' or 'potential for EMI in the engine harness' and instructs technicians to replace the hose assembly AND install a new external wiring harness jumper.

Mechanic-Grade Diagnostic Values

• Brake Booster Pressure Sensor - 5V Reference Voltage — expected: 4.9V to 5.1V at the sensor connector with key on, engine off.. Failure: Voltage below 4.9V or absent indicates a problem with the ECM or the wiring from the ECM.
• Brake Booster Pressure Sensor - Signal Voltage — expected: Approximately 5V with key on, engine off (no vacuum). Voltage drops toward 0.5V as vacuum increases (engine running). The signal should vary smoothly as the brake pedal is applied and released.. Failure: A constant low reading below 0.3V for 2 seconds will set the code. A stuck or erratic voltage that doesn't respond to vacuum changes indicates a faulty sensor.
• DTC Set Condition — expected: Normal operating voltage is above 0.3V.. Failure: The ECM will store P0557 if the sensor output voltage is 0.3V or less for 2 seconds.

Scan Tool Commands That Help

• GM GDS2 (Global Diagnostic System 2): Vehicle DTC Information / Module Scan — To perform a full vehicle scan to see if other modules have stored codes related to the brake system or communication, which can aid in diagnosis.
• GM GDS2 (Global Diagnostic System 2): Data Display / Current Data — To monitor the live data parameter for the 'Brake Booster Pressure Sensor' in volts or kPa. This allows the technician to observe the sensor's behavior in real-time as the engine runs and the brake is applied, which is the primary method to confirm a faulty sensor.

Wiring & Ground Locations

• B19B Brake Booster Vacuum Sensor Connector — On the black plastic vacuum pipe/hose assembly that runs from the engine to the brake booster on the driver's side of the engine bay.. This is the main connector for the sensor. TSB 20-NA-071 identifies poor terminal tension at this connector as a primary cause of P0557.
• ECM Connector X1 (Blue) — At the Engine Control Module (ECM).. This connector contains the three wires for the brake booster sensor circuit. Per TSB 20-NA-071, for the LFY V6 engine, the pins are: Pin 65 (Signal, YE/VT wire, Circuit 6030), Pin 45 (5V Reference, YE/RD wire, Circuit 6031), and Pin 44 (Ground, BK/YE wire, Circuit 6032).
• Ground G103 — Located at the left rear of the engine compartment on the cowl, above the brake booster.. This is a major ground point for the Body Control Module (BCM) and Engine Control Module (ECM). A poor ground here can cause a wide range of electrical issues, including incorrect sensor readings. While not the most likely cause for this specific code, it's a critical location to check for corrosion or looseness if wiring issues are suspected.

OEM Part Supersession History

• 84298056, 84341899, 84396439 → 85572832 — The original sensors were prone to failure from internal circuit board faults and inconsistent in-rush current during engine start/stop events. The newer part numbers, culminating in 85572832, contain a more robust sensor to better withstand these conditions.
• N/A → 84802275 (Jumper Harness) — This part was created by GM as a specific fix for TSB 20-NA-071 to address wiring issues like poor terminal tension and potential EMI in the factory engine harness. It is not a replacement but an additional part to be installed alongside the new sensor hose.
  Heads up: This jumper harness is specifically designed to connect between the new sensor and the ECM X1 connector, bypassing the original harness wires for the sensor circuit.