P0558 on 2020-2021 Cadillac XT6: Brake Booster Pressure Sensor Circuit High

What's Unique About the 2020-2021 Cadillac XT6

The Cadillac XT6, along with other GM vehicles on the C1XX platform (like the XT5, Chevy Traverse, and Buick Enclave), is known for this specific issue. The problem is so common that General Motors issued Technical Service Bulletin (TSB) #20-NA-071. The TSB explains that the root cause is often not the sensor itself, but rather "poor terminal tension at the B19B Brake Booster Vacuum Sensor" or "potential for EMI [electromagnetic interference] in the engine harness negatively impacting the signal". The original harness routing near components like ignition coils can induce false high voltage signals. Therefore, simply replacing the sensor often fails to fix the problem; the official repair requires a new, rerouted jumper harness as well.

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.
• Brake pedal is hard to press or requires significantly more effort.
• Intermittent loss of power brake assist.
• Increased stopping distance.
• Hissing sound from the brake booster area.

Most Likely Causes

1. Faulty Wiring Harness (EMI or Poor Terminal Tension) 🔴 High Probability This is a well-documented issue covered by GM Technical Service Bulletin #20-NA-071. The engine harness design can allow electrical interference (EMI) or have poor connector pin tension at the B19B sensor, leading to incorrect voltage readings.
   How to confirm: Confirming the presence of DTC P0558 along with the 'Service Power Brake Assist' message is the primary indicator. A technician will follow the diagnostic procedure outlined in TSB #20-NA-071, which is the presumptive cause if the repair has not been previously performed.
   Typical fix: Install a new brake booster vacuum hose/sensor assembly and a new, separate external wiring harness jumper to isolate the sensor signal from interference, as per the TSB. This involves cutting the old sensor wires and splicing the new harness into the ECM connector.
   Est. part cost: $75-$150
2. Failed Brake Booster Pressure Sensor 🟡 Medium Probability While the TSB points to wiring, the sensor itself can fail. The sensor is integrated with the vacuum hose assembly. An internal short or circuit board fault in the sensor can cause a constant high voltage signal. The TSB procedure replaces this part alongside the harness to ensure a complete repair.
   How to confirm: If the TSB's wiring fix doesn't resolve the issue, or if a technician tests the sensor's output voltage directly and finds it is stuck high despite having proper power and ground, the sensor is faulty. However, due to the TSB, it's standard practice to replace both components.
   Typical fix: Replace the brake booster vacuum hose and sensor assembly.
   Est. part cost: $75-$150

Rare But Worth Checking

• Damaged Vacuum Line: A crack or leak in the vacuum line between the engine and the brake booster can affect sensor readings, though it's less likely to cause a 'circuit high' electrical fault compared to a 'circuit low' or performance code.
• Powertrain Control Module (PCM) Fault: In very rare cases, the fault may lie within the PCM's internal circuitry. This should only be considered after all other possibilities (wiring, sensor) have been definitively ruled out by a professional.

Diagnosis Steps

1. Verify the code P0558 is present using an OBD-II scanner.
2. Check for the presence of a "Service Power Brake Assist" message on the driver information center.
3. Inspect the brake booster vacuum hose and the sensor's electrical connector (B19B) for any visible damage, corrosion, or loose connections.
4. Consult GM Technical Service Bulletin #20-NA-071 for the specific diagnostic and repair procedure.
5. Test the sensor circuit for proper voltage and ground. With the ignition on, the sensor should have a 5-volt reference and a good ground connection.
6. Measure the signal wire voltage. A reading that is stuck high (typically near 5 volts, or above 4.8V) confirms the 'Circuit High' condition.
7. Follow the TSB recommendation: Cut the old harness wires, tape them off, and install the new external wiring harness jumper. Replace the brake booster vacuum hose/sensor assembly.
8. Clear the DTCs and perform a road test, applying the brakes several times to ensure the fault does not return.

Parts You'll Likely Need

• Brake Booster Vacuum Hose with Sensor (OEM #84744220) — This assembly contains the sensor that fails or provides the faulty signal. It is replaced as part of the TSB repair procedure. Part number is an example for a related platform (Acadia) and should be verified by VIN.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $75-$150
  Aftermarket price range: $60-$120
• Brake Booster Vacuum Sensor Jumper Wiring Harness (OEM #84868339) — This is the specific part called for in TSB #20-NA-071 to fix the electrical interference and/or poor connection issue that causes the code. The part number is cited for the V6 Traverse, XT6, and other platform mates.
  Trusted brands: GM Genuine Parts
  OEM price range: $25-$50

Related Codes That Often Appear With This One

• P0557 — P0557 is for 'Brake Booster Pressure Sensor Circuit Low'. Both codes are related to the same sensor and are mentioned together in TSB #20-NA-071, indicating they stem from the same root cause of wiring and sensor issues.

Technical Service Bulletins (TSBs) & Recalls

• 20-NA-071: Provides repair information for a Service Engine Soon Lamp and/or Service Power Brake Assist message with DTC P0557 or P0558 stored. Identifies the cause as EMI or poor terminal tension.
• 21-NA-045: A related TSB for some platform mates (like the GMC Acadia) that also addresses P0558, but focuses on issues caused by the routing of the power brake booster vacuum pipe.

Platform-Specific Known Issues

• A known issue documented in GM Technical Service Bulletin #20-NA-071 describes how electrical interference (EMI) or poor terminal tension in the wiring harness can cause P0558 to set. The official fix involves installing a new, rerouted harness for the sensor and replacing the vacuum line/sensor assembly.

Mechanic-Grade Diagnostic Values

• Brake Booster Pressure Sensor Signal Voltage (Scan Tool) — expected: Approximately 4.5V - 5.0V with engine off (no vacuum), dropping toward 0.5V - 1.0V with the engine running at idle (high vacuum).. Failure: A signal voltage that is stuck high (e.g., above 4.8V for more than 2 seconds) will trigger the P0558 code.
• Brake Booster Pressure Sensor 5V Reference Voltage — expected: 4.9V - 5.1V with ignition on, engine off, measured at the sensor connector.. Failure: Voltage significantly below 4.9V or above 5.1V suggests an issue with the ECM's reference circuit or a short/open in the harness.

Scan Tool Commands That Help

• GDS2 / Techline Connect: Brake System Control Module Programming and Setup — A related TSB (20-NA-073) for other brake system DTCs on the 2020 XT6 requires reprogramming the Brake System Control Module. While not explicitly required for the P0558 TSB fix, it's a relevant dealer-level function that may be used if other brake system codes are present.

Wiring & Ground Locations

• B19B Brake Booster Vacuum Sensor — Mounted on the power brake booster vacuum hose assembly in the engine compartment, between the intake manifold and the brake booster.. This is the sensor that provides the signal to the ECM. TSB #20-NA-071 identifies poor terminal tension at this sensor's connector as a primary cause of the code.
• K20 Engine Control Module (ECM) Connector X1 — On the Engine Control Module (ECM) in the engine bay.. This is the specific connector where the TSB #20-NA-071 wiring repair is performed. The new jumper harness pins are inserted here after the old wires are removed.
• ECM X1, Pin 45 (5V Reference) — Pin 45 of the X1 connector at the ECM. Original wire is Yellow/Red (YE/RD).. This is the 5-volt reference circuit from the ECM to the sensor. The new harness wire (tagged #6271) is installed here.
• ECM X1, Pin 65 (Signal) — Pin 65 of the X1 connector at the ECM. Original wire is Yellow/Violet (YE/VT).. This is the signal circuit from the sensor back to the ECM. The new harness wire (tagged #6270) is installed here.
• ECM X1, Pin 44 (Low Reference/Ground) — Pin 44 of the X1 connector at the ECM. Original wire is Black/Yellow (BK/YE).. This is the ground circuit for the sensor. The TSB requires cutting the original wire and splicing the new harness wire (#6272) along with the original harness-side wire to the ECM-side of the wire.

Real Owner Repair Stories

• traverseforum.com user post (2019 Chevrolet Traverse (platform-mate)) — 'Service Power Brake Assist' message, Check Engine Light, and a hard brake pedal.
  ❌ Tried (didn't work) Owner took the vehicle directly to the dealer without attempting other fixes.
  ✅ 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 and replacing the sensor/hose assembly. The repair was covered under warranty.

Model Year Variations Within This Range

• 2020-2021: The Cadillac XT6 uses the LGX/LFY V6 engine. The ECM pin locations for the B19B sensor are at connector X1, pins 44, 45, and 65. Other GM vehicles on the same platform with a 4-cylinder (LCV) engine use different ECM connectors (X1 and X3) and pin locations for the same TSB repair.