Comprehensive diagnostic guide for OBD-II code P0558

What This Code Means

• Primary definition: P0558 is described in an open-source code repository as a Brake Booster Pressure Sensor High Input, i.e., the brake servo
• Context in OBD-II framework: P0558 is a DTC within the Powertrain Codes (P0xxx) category, which covers many engine and drivetrain control systems monitored by the OBD-II system.

What This Code Means

• The brake booster pressure sensor monitors the vacuum/pressure within the brake booster (servo). A high input reading from that sensor is interpreted by the PCM/ECU as an abnormal condition. The ECM/PCM may set the MIL and P0558 when the sensor reading exceeds expected ranges consistently or during fault conditions detected in the sensor circuit.

User-facing symptoms you may observe

• MIL/Check Engine Light illuminated or flashing, with P0558 stored in the PCM.
• Changes in brake pedal feel: reduced brake boost or a hard pedal due to incorrect brake booster signaling.
• Possible vacuum-system symptoms: audible vacuum leak sounds, or inconsistent brake assist behavior.
• Potential for related codes to appear (e.g., P0550-P0557 family codes) if the system detects broad brake-boost sensor issues or related circuitry problems.
  Note: Symptoms can vary by vehicle and may appear alongside other P055x codes.

Probable Causes

• Most likely: Faulty brake booster pressure sensor itself (sensor failure or out-of-calibration condition). 40-50%
• Commonly implicated: Wiring harness/connectors to the brake booster pressure sensor or its PCM input damaged, corroded, or shorted (to ground or +12 V). 20-30%
• Vacuum-side issues: Brake booster vacuum system problems such as a vacuum hose leak, faulty check valve, or booster internal leak that causes abnormal sensor readings. 15-25%
• PCM/ECU input or software issue: occasional misread or calibration/software fault in the PCM that interprets the analog signal incorrectly. 5-10%
• Other possibilities: Intermittent harness ground issues, external electrical noise, or a combination of small faults that together trigger the code. 5-10%

Important safety notes

• Brake booster/boosted-brake assist is integral to safe braking. If you suspect a brake-boost or vacuum-system fault, treat the condition as safety-critical. Do not perform field tests that could disable braking function in a way that endangers the vehicle occupants. Always work in a controlled environment and use proper PPE.

Tools and data you'll typically need

• OBD-II scan tool capable of live data/POI monitoring and freeze-frame data.
• Digital multimeter (DVOM) and/or oscilloscope for sensor signal and power/ground checks.
• Vacuum gauge and/or a smoke machine for vacuum-system leakage diagnostic.
• Service information for the particular vehicle (to confirm sensor pinout, sensor supply voltage, expected signal range, wiring harness color codes, and service procedures).
• General hand tools (screwdrivers, sockets) for sensor and connector access.
• The diagnostic plan may require temporary battery-off or disconnection steps per vehicle-specific service procedures.

Recommended diagnostic plan (step-by-step)

1) Confirm and document

• Use the scan tool to confirm P0558 is present and note any related codes (e.g., P0550-P0557). Read freeze-frame data to determine engine load, RPM, throttle position, and other parameters at the time of fault.
• Check for vehicle-specific service bulletins that reference brake booster pressure sensor or related brake-boost hardware.

2) Visual inspection

• Inspect the brake booster and its vacuum supply line(s) for damage, cracks, or disconnections.
• Inspect the vacuum check valve (one-way valve) and hose connections for leaks or contamination.
• Inspect the brake booster pressure sensor electrical harness and connector for secure fit, corrosion, bent pins, or damaged insulation.

3) Vacuum-system integrity test

• With the engine running (and/or as specified by the factory procedure), perform a vacuum test or smoke test to check for leaks in the brake booster vacuum line, check valve, or booster housing. A significant vacuum leak can cause abnormal sensor readings or boost behavior.
• If tools indicate a leak, address the vacuum system (hose replacement, valve replacement, booster repair/replacement) and recheck.

4) Sensor data verification (live data)

• Monitor the brake booster pressure sensor signal in real time (as available through the scan tool) while operating the engine at idle and at various loads.
• Note whether the signal is stable, rises to high values, or fluctuates unexpectedly. Verify that the sensor is receiving proper power (often 5 V reference), ground, and a clean signal path.
• Compare sensor readings to the vehicle-specific specification. If the signal is consistently at the high end or out of range, suspect the sensor or signal wiring.

5) Wiring and connector checks

• With the ignition on and engine off, perform a continuity/ resistance check on the sensor wiring from the connector to the PCM (as per service manual). Look for shorts to voltage, shorts to ground, or open circuits.
• Inspect for signs of chafed wiring, exposed conductors, or moisture intrusion in connectors.

6) Sensor testing

• If permitted by the service manual, perform a controlled test: disconnect the brake booster pressure sensor and observe PCM/ECU behavior. If the signal goes away or the MIL codes change, the sensor or its circuit is implicated.
• If you have the ability and the vehicle allows, bench-test or substitute with a known-good sensor to verify whether the fault follows the sensor.

7) Related system checks

• Inspect for any software/PCM calibration updates that affect sensor interpretation. If applicable, check for TSBs or updates from the OEM.
• Check for other fault codes that indicate broader brake or electrical issues.

8) Fault isolation and repair

• If the brake booster pressure sensor is confirmed faulty, replace the sensor (and related weatherproof seals/connectors as required by the vehicle).
• If wiring is damaged, repair or replace the wiring harness and repair connectors as needed; ensure grounds are clean and secure.
• If a vacuum leak or faulty booster/check valve is found, repair or replace the affected components (booster, check valve, or hose) and recheck the system.
• Clear codes and perform a road test to verify that P0558 does not reappear and that brake boost function returns to normal.
• If the fault remains after sensor and vacuum-system repairs, consider PCM input integrity or software recalibration as a last resort per OEM guidance.

Post-Repair Verification

• Re-scan to confirm P0558 is cleared and that no new codes are present.
• Drive the vehicle through typical operating conditions to verify brake assist behavior under various loads and speeds.
• Confirm that live sensor data now falls within expected ranges and that the brake booster vacuum system maintains proper operation during multiple cycles of pedal application.
• Ensure readiness monitors (if applicable) return to ready state.

Notes on code interpretation and possible ambiguities

• The available sources identify P0558 as a brake booster pressure sensor high input in an open-source definition. Wikipedia's OBD-II references place DTCs in the broader context of Powertrain Codes and diagnostic troubleshooting, but do not provide a vehicle-specific P0558 definition. Given the sources, P0558 is interpreted as brake-boost related. If a particular OEM lists P0558 as in their documentation, rely on the OEM service data for the definitive definition and testing procedure for that model. In cases of conflict, follow the OEM-specific diagnostic information as primary and treat the general brake-boost interpretation as a guide.

Probability-based quick-reference (summary)

• Sensor itself faulty: 40-50%
• Wiring/connector issue: 20-30%
• Vacuum system (booster, check valve, hoses) fault: 15-25%
• PCM/software/communication issue: 5-10%
• Other/combined issues: 5-10%

Cited context from sources

• OBD-II diagnostic framework and powertrain code classification. These sources provide the framework for how P0558 fits into P0xxx codes and how DTCs are used in OBD-II systems.
• Specific code interpretation for P0558 in an open-source repository: (High input signal from the brake booster pressure sensor). This defines P0558 as a brake-boost related sensor issue.
• General diagnostic logic for powertrain and emissions-DTC context in the OBD-II literature (to support broad steps and safety considerations).

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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