Error Code P0559 is defined as Brake Booster Pressure Sensor Circuit Intermittent. This means there is a problem with the brake booster pressure sensor, likely caused by vacuum leaks or other problems with the brake booster sensor itself.

This code is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-system and brake booster pressure sensor. It’s particularly common, but not limited to Acura, Alfa Romeo, Chevrolet, Dodge, Ford, Hyundai, Nissan, Pontiac, Subaru, etc. Though generic, specifications on the definition, troubleshooting, and repairs may vary from year, make, model, and powertrain configurations of the vehicle.

The job of the brake booster pressure sensor circuit is to monitor the amount of pressure (vacuum) applied to the power brake booster. This process is controlled by the PCM to allow the vehicle to adapt to various braking situations and adjust the pressure needed by the vehicle to assist the operation of the brake pedal. This process is crucial for the power brakes to work efficiently and safely, and safely slow down or stop the vehicle with minimum amount of foot pressure being applied to the vehicle.

Error Code P0559 is one of the many possible codes associated with the brake booster pressure sensor circuit. When this appears, it means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has determined an error indicating that the brake booster pressure sensor circuit is out of range, or not running properly due to abnormal feedback.

When the brake booster pressure sensor circuit has a voltage reading higher than expected, then this code is set.

This code is set by the PCM when the brake booster pressure sensor circuit has an intermittent output voltage signal problem.

Specific situations determine the code which the PCM activates. Other related brake booster pressure sensor circuit error codes include:

For some vehicles, this code only appears when applying the brakes. While for other makes, the vehicle may require multiple ignition cycles before this code pops up.

Common Symptoms

When this code appears, it will illuminate the Check Engine light, or flash it when braking. Other common symptoms include:

  • Harder and more resistant brake pedal
  • Check brake light flashes or stays illuminated

In some cases, there may be no noticeable drivability and performance symptoms.

Possible Causes

  • Vacuum leaks in vacuum brake booster
  • Defective brake booster pressure sensor
  • Damaged booster vacuum hoses
  • Damaged vacuum brake booster supply hose valve
  • Corroded, damaged, or loose electrical components or circuits
  • Low level of power steering fluid
  • Defective steering pump (leads to failure of hydraulic brake)
  • Defective brake booster (in rare cases)

How to Check

As with most trouble codes, it is best to refer with your TSB (Technical Service Bulletin) when troubleshooting this code.

Start by locating all components related to the Brake Booster Pressure Sensor Circuit. This includes the sensor itself, the brake booster, vacuum lines, wirings, and connectors to the PCM. As soon as you find these components, inspect the connectors and wirings for signs of damages, such as scraping, bare wires, burn spots, rubbing, etc. Also, check the connectors for signs of corrosions buildup, or damaged pins. Vacuum lines must be checked for weather cracks, leaks, and overall serviceability level.

Advanced Steps

Next step for diagnosis is usually vehicle specific and may require the appropriate and advanced equipment to be conducted properly. This includes the use of a digital multimeter, and specific technical references for the vehicle, a voltage requirement is usually based on the make, model, and specific year of the vehicle.

Circuit Checks

As said earlier, the voltage requirement varies for every vehicle, the configuration of the brake booster pressure sensor circuit, and all components incorporated into the system. Thus, tech data must be referenced to obtain the right voltage range from the brake booster pressure sensor and the appropriate troubleshooting sequence.

The correct voltage input to a sensor without output voltage is usually a sign of internal failure.

If this process confirms the absence of power source or ground, then you should conduct a continuity test to check the condition of the connectors and wirings. This test is always performed with power removed from the circuit, while normal readings should start at 0 ohms of resistance unless otherwise specified by the technical data from the TSB.

Resistance or no continuity means there’s a faulty wiring or connector issue that is shorted or open, which must be repaired, if not replaced.

How to Fix

Depending on your diagnosis, common repairs for this code include:

  • Replacement of brake booster pressure sensor
  • Repair or replacement of faulty electrical components in the brake booster (connectors and wires)
  • Replacement of leaking vacuum hoses
  • Replacement of damaged hoses and valves in brake booster system
  • Replacement of brake booster

Severity for this problem depends on the specific malfunction. Usually, though, this condition progress over time if not corrected, which could pose a safety risk. Thus, this code requires immediate attention.