P0559 — Brake Booster Pressure Sensor Circuit Intermittent #

Note: The description for DTC P0559 may vary depending on vehicle manufacturer.

• Brake Booster Pressure Sensor Circuit Intermittent (Generic/most OEMs)
• Brake Booster Pressure Sensor Circuit Intermittent Signal (Volkswagen/Audi)
• Intermittent Signal from Brake Booster Pressure Sensor (Ford/Lincoln)
• Brake Booster Pressure Sensor Circuit Sporadic (GM/Chevrolet)

Summary of DTC P0559 #

The P0559 code indicates that the powertrain control module (PCM) or engine control module (ECM) has detected an intermittent problem in the brake booster pressure sensor circuit. The brake booster is a key component that assists with braking force by using vacuum or hydraulic pressure, and the pressure sensor monitors the status inside the booster. An intermittent circuit means the sensor signal is unstable — it may drop out or fluctuate unexpectedly. This can compromise braking performance and may trigger warning lights or alter brake assist functions. Prompt diagnosis is crucial, as it impacts both safety and drivability.

Frequent Symptoms for DTC P0559 #

An intermittent issue with the brake booster pressure sensor circuit can manifest with several noticeable symptoms. Drivers may experience inconsistent brake pedal feel, or the vehicle may illuminate the brake warning or check engine light. Some systems may reduce brake assist as a precaution, leading to a firmer or less responsive pedal. Occasionally, there may be no obvious symptoms apart from a stored trouble code.

• Brake warning light or Check Engine Light (CEL) illuminated
• Inconsistent or hard brake pedal
• Reduced brake assist (longer stopping distances)
• Intermittent loss of vacuum assist
• Unusual hissing noise from brake booster area (less common)

Probable Causes for DTC P0559 (in order of likelihood) #

Several issues can trigger an intermittent circuit code for the brake booster pressure sensor. The most common causes involve electrical or wiring faults, but sensor failure or issues with the brake booster itself can also be at play. Here’s a breakdown, from most to least common:

1. Damaged or corroded wiring/connectors at the brake booster pressure sensor
2. Loose, dirty, or poor electrical connections
3. Faulty brake booster pressure sensor (internal failure)
4. Vacuum leaks affecting sensor operation
5. Faulty PCM/ECM (rare)

Step-by-Step Diagnosis for DTC P0559 #

A systematic approach is essential for diagnosing intermittent circuit faults. Intermittent issues can be elusive, so patience and attention to detail are key. Follow these steps:

• 1. Use a scan tool to confirm the presence of P0559 and check for related codes.
• 2. Inspect the brake booster pressure sensor and associated wiring for visible damage, corrosion, or loose connections.
• 3. Wiggle test: With the engine running, gently move wiring harnesses and connectors to the sensor while monitoring live data for signal drops or glitches.
• 4. Check for vacuum leaks around the brake booster and the vacuum supply hose.
• 5. Test sensor output with a multimeter or scan tool; compare values to OEM specifications.
• 6. If wiring and sensor check out, inspect related fuses and relays.
• 7. As a last resort, consider PCM/ECM evaluation if all other components are confirmed good.

Possible Repairs for DTC P0559 #

Repair strategies should directly address the root cause identified during diagnosis. Start with the simplest fixes, as electrical issues are often the culprit.

• Repair or replace damaged wires/connectors in the sensor circuit
• Clean and secure all electrical connections at the sensor and harness
• Replace the brake booster pressure sensor if it fails testing
• Fix any vacuum leaks in the brake booster or its vacuum supply
• Replace the PCM/ECM (only after all other causes have been ruled out)

Is It Safe to Drive With Code P0559? #

Driving with code P0559 is not recommended until the issue is properly diagnosed and repaired. While some vehicles may only display a warning light with no immediate loss of brake assist, others can experience a reduction in braking performance, especially under emergency conditions. Brake system reliability is critical for safety — intermittent assist or sensor readings can increase stopping distances or result in unpredictable pedal feel. Prioritize repair to avoid unnecessary risk.

Vehicles Most Commonly Affected by DTC P0559 #

While P0559 is a generic OBD-II code, it is most frequently reported on vehicles that use an electronically monitored brake booster pressure system. This includes, but is not limited to:

• Volkswagen and Audi models (e.g., Passat, Jetta, A4, Q5)
• Ford and Lincoln vehicles (e.g., Fusion, Taurus, MKZ)
• Chevrolet and GM vehicles (e.g., Malibu, Cruze, Equinox)
• Some Japanese models with advanced brake assist (e.g., select Nissan, Toyota, Honda)

Common Mistakes & Best Practices for DTC P0559 #

Diagnosing intermittent electrical faults can be tricky. Technicians and DIYers often make certain missteps that prolong repairs or cause repeat failures. Avoid these pitfalls:

• Replacing the sensor without checking wiring or connectors
• Overlooking vacuum leaks that can skew sensor performance
• Neglecting to use OEM specifications for testing sensor output
• Failing to perform a thorough wiggle test for intermittent faults
• Ignoring related codes that may help pinpoint the root issue

FAQ for DTC P0559 #

Still have questions? Here are quick answers to common concerns:

• Q: Can I reset the code and see if it comes back?
  A: You can clear the code, but if the underlying issue isn’t fixed, it will almost certainly return. Always diagnose the cause first.
• Q: Is this an expensive repair?
  A: Costs vary. If it’s just a wiring or connector issue, repairs are usually inexpensive. Replacement sensors or brake boosters can be more costly, especially on European models.