P0558 on 2019-2021 Chevrolet Blazer: Brake Booster Pressure Sensor Circuit High Causes and Fixes

What's Unique About the 2019-2021 Chevrolet BLAZER

The 2019-2021 Blazer is built on GM's C1XX platform, which it shares with vehicles like the 2018+ Chevrolet Traverse, 2017+ GMC Acadia, 2018+ Buick Enclave, and Cadillac XT5/XT6. This platform has a known issue where the brake booster pressure sensor circuit is susceptible to electromagnetic interference (EMI), suspected to be from proximity to ignition coils. GM acknowledged this with a specific Technical Service Bulletin (TSB #20-NA-071) that outlines a unique repair involving a new, separate wiring harness, rather than just a simple sensor replacement.

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.
• Increased effort required to stop the vehicle.
• Longer than normal stopping distances.
• Intermittent loss of power assist during a stop.

Most Likely Causes

1. Wiring Harness Interference / Poor Terminal Tension 🔴 High Probability As identified in TSB #20-NA-071, the original engine harness design allows for electrical interference (EMI) or poor terminal tension at the sensor connector, causing incorrect high voltage readings. The wiring's routing near other components is the source of the interference.
   How to confirm: A technician will confirm the P0558 code and check for the conditions described in TSB #20-NA-071. Diagnosis involves inspecting the wiring harness and testing the circuit's voltage. The ECM sets the code if the signal is 4.8V or higher for 2 seconds.
   Typical fix: The official GM fix is to replace the power brake booster vacuum hose/sensor assembly and install a new, separate wiring harness (jumper) that splices into the ECM connector. The old sensor wiring is cut and abandoned. This new harness shields the signal from interference.
   Est. part cost: $75-$150
2. Faulty Brake Booster Pressure Sensor 🟡 Medium Probability While the wiring is the most common culprit, the sensor itself can fail internally due to a circuit board fault, causing it to send a constant high voltage signal. TSB #20-NA-071 recommends replacing the sensor/hose assembly as part of the complete repair, even if the main fault is the wiring.
   How to confirm: If the wiring and connections are confirmed to be good, the sensor's output voltage can be tested directly with a multimeter or scan tool. A reading that is stuck high (typically near 5 volts) regardless of engine vacuum indicates a failed sensor.
   Typical fix: Replace the brake booster pressure sensor. On this vehicle, it is serviced as an assembly with the vacuum hose. The TSB should still be performed to prevent a recurrence.
   Est. part cost: $50-$100

Rare But Worth Checking

• Vacuum Leak: A significant vacuum leak in the brake booster or the vacuum line leading to it could theoretically cause the sensor to read incorrectly. However, this is less common for a 'circuit high' code than for a performance-related code like P0556. A hissing sound near the booster is a key symptom.
• Powertrain Control Module (PCM) Fault: In very rare cases, the fault may lie within the PCM itself, where it incorrectly processes the signal from the sensor. This should only be considered after all other possibilities (wiring, sensor, TSB procedure) have been definitively ruled out.

Diagnosis Steps

1. Verify the P0558 code is present using an OBD-II scanner.
2. Check for the presence of other codes, especially P0557.
3. Consult GM Technical Service Bulletin #20-NA-071, as it directly addresses this code for this vehicle and its platform mates. This is the most critical step.
4. Inspect the wiring harness and connector going to the brake booster pressure sensor for any visible damage, chafing, or corrosion.
5. With the key on and engine off, use a scan tool to monitor the brake booster pressure sensor voltage. A reading stuck at or near 5 volts (GM specifies a threshold of 4.8V) points towards a circuit high condition.
6. Follow the detailed procedure in TSB #20-NA-071, which involves testing specific pins at the ECM connector, installing the new jumper harness, and replacing the sensor/hose assembly.
7. The TSB specifies which ECM terminals to use based on the engine (e.g., for LGX/LFY V6 engines, it involves terminals 65, 45, and 44 in the X1 connector).
8. After the repair, clear all DTCs and perform a test drive to ensure the fault does not return.

Parts You'll Likely Need

• Brake Booster Pressure Sensor Wiring Harness Jumper (OEM #84868339) — This is the specific part GM released to fix the electrical interference issue described in TSB #20-NA-071 for V6 models. It creates a new, shielded circuit for the sensor signal.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $40-$70
  Aftermarket price range: $30-$50
• Brake Booster Vacuum Hose with Sensor (OEM #84529302) — The TSB recommends replacing the sensor and hose assembly along with installing the new harness to ensure a complete repair. This part number is commonly associated with the V6 Blazer for this repair.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $50-$100
  Aftermarket price range: $40-$80

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 circuit and are mentioned together in TSB #20-NA-071 as resulting from the same root cause of electrical interference or poor connections.

Technical Service Bulletins (TSBs) & Recalls

• 20-NA-071: Addresses a 'Service Power Brake Assist' message with DTC P0557 or P0558. It identifies the cause as electrical interference or poor terminal tension and provides a specific wiring and sensor replacement procedure, including ECM pinout locations.

Platform-Specific Known Issues

• TSB #20-NA-071 Wiring Fix: A known issue documented in TSB #20-NA-071 describes how electrical interference (EMI) or poor terminal tension in the engine harness can cause DTC P0558. The official fix requires installing a new, separate wiring harness (jumper) and replacing the sensor/hose assembly. The new harness is routed away from interference sources and pinned directly into the ECM.

Mechanic-Grade Diagnostic Values

• Brake Booster Pressure Sensor Signal Voltage (Key On, Engine Off) — expected: ~5.0V (no vacuum). Failure: A reading stuck at or near 5.0V (GM specifies >4.8V) even when the engine is running and producing vacuum.
• Brake Booster Pressure Sensor Signal Voltage (Engine Running, Brake Pedal Released) — expected: Voltage should drop significantly from the KOEO reading, typically towards ~0.5V - 1.0V, as engine vacuum is applied.. Failure: Voltage remains high (>4.8V), indicating the sensor is not reading the vacuum.
• Sensor 5V Reference Circuit — expected: A steady 5.0V. Failure: Voltage that is not present, low, or unstable would indicate a wiring or ECM issue, but is not the typical cause for a 'Circuit High' code.

Scan Tool Commands That Help

• GDS2 / Professional Scan Tool: Live Data Monitoring (Brake Booster Pressure Sensor PID) — This is the primary method to diagnose P0558. Monitor the sensor's voltage or pressure reading with the key on/engine off, and then with the engine running. A reading that does not change or stays high when vacuum is applied confirms the fault condition described in TSB #20-NA-071.
• GDS2 / High-End Bidirectional Scanner: Module Status / Communication Test — In the rare event that the TSB procedure does not resolve the issue, this function can be used to verify that the ECM/BCM is properly communicating on the CAN bus. A communication fault (e.g., U-codes) could potentially masquerade as a sensor fault.

Wiring & Ground Locations

• Engine Control Module (ECM/PCM) — On the 2019-2021 Blazer and its platform mates, the ECM is located in the front driver's side wheel well, behind the felt liner.. The TSB #20-NA-071 repair requires running a new jumper harness from the brake booster pressure sensor directly to the ECM. Technicians need to access the ECM connectors (specifically X1 on V6 models) to de-pin the old circuit and install the new one.
• G108 (Example Engine Bay Ground) — On similar GM truck platforms, a key ground point (G108) is located on the firewall in the left rear corner of the engine compartment, often above the power brake booster.. While the primary cause of P0558 is EMI or terminal tension, a poor engine or chassis ground could cause various electrical issues. Verifying that major grounds near the brake booster and ECM are clean and secure is a good secondary check.

Real Owner Repair Stories

• Go-Parts article citing an owner report (2017-2021 Cadillac XT5 (C1XX Platform Mate)) — 'Service Power Brake Assist' warnings.
  ❌ Tried (didn't work) Initial diagnosis pointed towards the common P0558 sensor/wiring issue.
  ✅ What actually fixed it The owner reported that replacing a weak AGM battery resolved the warnings. The battery had passed a simple test but failed a more thorough extended load test, suggesting that unstable system voltage can sometimes trigger this sensitive circuit.

OEM Part Supersession History

• TSB #20-NA-071 → 20-NA-071A, 20-NA-071B — The TSB has been updated, likely to include additional vehicle models, clarify procedures, or update part numbers.
  Heads up: Always use the latest version of the TSB for the most current part numbers and procedures.
• 84868339 (Jumper for Blazer/Traverse V6) → N/A — This part number appears to be specific to the LGX V6 engine in certain models.
  Heads up: Note that other platform mates may use a different jumper harness part number, for example, the Buick Enclave is listed with part #84807853. Always verify the part number by VIN and the latest TSB.

Model Year Variations Within This Range

• 2017-2021 (Platform Wide): A related TSB, #21-NA-045, also mentions P0558 and other brake booster codes, pointing to issues with the routing of the power brake booster vacuum pipe itself. This indicates the entire vacuum assist system on this platform is sensitive and has multiple potential failure points addressed by GM service documents.