P0558 on 2018-2021 Buick Enclave: Brake Booster Pressure Sensor Circuit High Fixes

What's Unique About the 2018-2021 Buick ENCLAVE

On this generation of the Buick Enclave and its platform-mates (like the Chevy Traverse and GMC Acadia), the P0558 code is a well-documented issue. 🎬 Watch: Understanding the P0558 circuit high trouble code. General Motors issued a specific Technical Service Bulletin (TSB 20-NA-071, later revised) to address it. The problem is often not just a faulty sensor, but electrical interference (EMI) or poor terminal connections in the wiring harness between the sensor and the engine computer. The original wiring is routed close to the ignition coils, which is the suspected source of the interference, necessitating a specific repair that involves installing a new, rerouted jumper harness.

Diagnostic Flowchart

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

Symptoms You May Notice

• "Service Power Brake Assist" message on the instrument cluster.
• Check Engine Light (Service Engine Soon Lamp) is on.
• Hard brake pedal, requiring much more effort to stop the vehicle.
• Increased stopping distance.
• Hissing noise from the engine bay or near the brake pedal, indicating a possible vacuum leak.
• Engine may stall when braking in some cases.

Most Likely Causes

1. Faulty Wiring Harness or Poor Connection 🔴 High Probability GM TSB #20-NA-071 specifically identifies poor electrical terminal tension or Electromagnetic Interference (EMI) in the engine harness as a primary cause. The original harness runs close to ignition coils, which can induce voltage spikes, causing the ECM to see a false 'high' signal.
   How to confirm: Inspect the wiring harness between the brake booster pressure sensor and the ECM for any signs of damage, corrosion, or loose connections. A technician would use a scan tool to see if the sensor voltage is stuck high (near 5V). Disconnecting the sensor should cause the voltage to drop to 0V; if it remains high, a short to voltage in the harness is confirmed.
   Typical fix: The official GM fix involves installing a new, separate wiring harness (jumper), GM Part #84807853, that reroutes the signal away from the source of interference. Often, the power brake booster vacuum hose, which includes a new sensor, is replaced at the same time.
   Est. part cost: $50-$150
2. Faulty Brake Booster Pressure Sensor 🟡 Medium Probability The sensor itself can fail internally, sending an incorrect high voltage signal to the ECM. This is a common failure point for this type of code across many vehicles. However, on this platform, the wiring is the more frequent culprit.
   How to confirm: With a diagnostic scan tool, monitor the live data from the sensor. If the voltage reads consistently high and does not change with brake application (and the wiring has been confirmed to be good by disconnecting it), the sensor is likely faulty.
   Typical fix: Replace the brake booster pressure sensor. On these models, the sensor is integrated into the brake booster vacuum hose assembly, GM Part #84732973.
   Est. part cost: $75-$125
3. Vacuum Leak ⚪ Low Probability
   How to confirm: Visually inspect all vacuum lines connected to the brake booster for cracks, brittleness, or loose connections. Listen for a hissing sound with the engine running, which can indicate a leak. A smoke machine can also be used to pinpoint the source of the leak.
   Typical fix: Replace the cracked or leaking vacuum hose.
   Est. part cost: $20-$60

Rare But Worth Checking

• Faulty Engine Control Module (ECM): This is very rare. The ECM should only be considered after all other possibilities, including the sensor, wiring (per TSB 20-NA-071), and vacuum system faults, have been exhaustively ruled out.

Diagnosis Steps

1. Use an OBD-II scanner to confirm P0558 is present and check for other codes, especially P0557.
2. Check for any relevant Technical Service Bulletins (TSBs). TSB 20-NA-071 (and its revisions) is directly applicable.
3. Visually inspect the brake booster pressure sensor and its electrical connector for any obvious damage, corrosion, or loose connections.
4. Inspect the vacuum hoses connected to the brake booster for any cracks, splits, or disconnections. Listen for a hissing sound with the engine running.
5. Using a scan tool with live data capability, monitor the voltage from the brake booster pressure sensor. A reading that is stuck high (typically near 4.9-5.0 volts) points to a circuit high fault.
6. If the reading is stuck high, disconnect the sensor's electrical connector. If the scan tool reading now drops to low (near 0 volts), the sensor is likely faulty. If the reading stays high, there is a short to power in the wiring harness that must be addressed.
7. Follow the specific diagnostic and repair procedure outlined in TSB 20-NA-071, which involves installing a new dedicated wiring harness (GM Part #84807853) and often replacing the vacuum hose/sensor assembly (GM Part #84732973).

Parts You'll Likely Need

• Brake Booster Pressure Sensor Wiring Harness (Jumper) (OEM #84807853) — TSB 20-NA-071 identifies a wiring issue as the root cause, and the official fix is to install this new, rerouted harness to prevent electrical interference from components like ignition coils.
  Trusted brands: GM Genuine Parts
  OEM price range: $50-$100
• Power Brake Booster Vacuum Hose with Sensor (OEM #84732973) — This assembly contains the brake booster pressure sensor. Per TSB 20-NA-071, this is often replaced along with the wiring harness to ensure a complete repair. The sensor itself can also fail.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $75-$125
  Aftermarket price range: $50-$90

Related Codes That Often Appear With This One

• P0557 — P0557 is 'Brake Booster Pressure Sensor Circuit Low'. Both P0557 and P0558 are mentioned together in TSB 20-NA-071 as being caused by the same wiring harness and terminal tension issue.

Technical Service Bulletins (TSBs) & Recalls

• 20-NA-071 / 20-NA-071A / 20-NA-071B: Provides a correction for the Service Engine Soon Lamp illuminated and/or Service Power Brake Assist message displayed, with DTCs P0557 or P0558 stored. It identifies the cause as potential EMI or poor terminal tension and details the installation of a new wiring harness and sensor/hose assembly.

Platform-Specific Known Issues

• TSB 20-NA-071 and Electrical Interference: GM Technical Service Bulletin #20-NA-071 (later updated to 20-NA-071A and 20-NA-071B) was issued for 2018-2021 Buick Enclave, Chevrolet Traverse, and other platform-mates to address DTCs P0557 and P0558. The bulletin points to electrical interference (EMI) or poor terminal tension in the wiring harness as the cause. The prescribed fix is not just to replace the sensor, but to install a new jumper harness (Part #84807853) that is routed away from the ignition coils to shield it from interference, and often replace the sensor/hose assembly (Part #84732973) as well.
• Owner Experience: Dealer Repair: A common owner experience involves the 'Service Power Brake Assist' light appearing, followed by a hard brake pedal. A forum member on traverseforum.com with a 2019 Traverse reported these exact symptoms. The dealer diagnosed the issue, referenced TSB 20-NA-071, and performed the repair by installing the new harness and sensor assembly. The repair was covered under the vehicle's bumper-to-bumper warranty.

Mechanic-Grade Diagnostic Values

• Brake Booster Pressure Sensor Signal Voltage (Scan Tool) — expected: Voltage should vary with brake pedal application and engine vacuum. A normal reading at idle with no brake application is typically much lower than 5V.. Failure: A reading stuck at or near 4.8V - 5.0V for 2 seconds or more, regardless of brake application, will trigger DTC P0558.
• Sensor 5V Reference Voltage at Connector — expected: Approximately 5.0V with Key On, Engine Off.. Failure: Significantly lower or higher voltage indicates a problem with the ECM or wiring.
• Sensor Ground Circuit at Connector — expected: Near 0V, with good continuity to a known good ground.. Failure: Voltage present on the ground line or high resistance (open circuit) indicates a grounding issue.

Scan Tool Commands That Help

• GM GDS2 (Global Diagnostic System 2): Live Data Monitoring for Brake Booster Pressure Sensor — This is the primary tool for diagnosis. It allows a technician to view the sensor's reported voltage in real-time to see if it is stuck high, confirming the fault condition.
• GM GDS2 / TIS2Web: Clear All DTCs — After completing the repair (installing the new harness and/or sensor), all DTCs must be cleared. A test drive should be performed to ensure the code does not return.

Wiring & Ground Locations

• ECM Connector X1 (Blue) — On the Engine Control Module (ECM) in the engine bay.. TSB 20-NA-071 requires de-pinning and re-pinning circuits at this connector. Specifically, circuits 6030 (Signal), 6031 (5V Ref), and 6032 (Low Ref/Ground) are moved from the main harness to the new jumper harness at pins 65, 45, and 44 respectively for the LGX/LFY V6 engines.
• G103 — Located at the left rear of the engine compartment on the cowl, often above the brake booster.. This is a major ground point for multiple modules including the Body Control Module (BCM) and Instrument Panel Cluster (IPC). While not the direct ground for the sensor (which grounds through the ECM), a poor G103 connection can cause widespread electrical issues that could potentially mimic sensor circuit problems.

Real Owner Repair Stories

• traverseforum.com user post (2019 Chevrolet Traverse (platform-mate to Enclave)) — 'Service Power Brake Assist' message, Check Engine Light, and a hard brake pedal.
  ❌ Tried (didn't work) Not specified; owner took the vehicle directly to the dealer.
  ✅ What actually fixed it The dealership diagnosed the vehicle and performed the repair outlined in TSB 20-NA-071. This involved installing the new jumper harness (Part #84807853) and replacing the brake booster vacuum hose/sensor assembly. The issue was resolved and covered under warranty.

OEM Part Supersession History

• Unknown → 84732973 (Hose/Sensor Assembly) — This is the part number called for in the TSB 20-NA-071 repair, likely updated to be more robust or compatible with the new harness fix.
• Unknown → 84807853 (Jumper Harness) — This part was created specifically to address the EMI issue described in TSB 20-NA-071.