Error Code P0557 is defined as Brake Booster Pressure Sensor Circuit Low, meaning there’s a problem with the brake booster pressure sensor circuit, which could be caused by a faulty sensor, broken circuits, or vacuum leaks.

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 by 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 a minimum amount of foot pressure being applied to the vehicle.

Error Code P0557 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 low output voltage for the PCM, then this code is set.

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:

  • Check brake light flashes or stays illuminated
  • Unresponsive brake pedal (pressing the brake pedal requires increase effort)
  • Softer brake pedal in some cases

Possible Causes

  • Defective one-way valve in the vacuum brake booster supply hose
  • Faulty brake booster pressure sensor
  • Damaged or worn vacuum lines
  • Faulty wiring
  • Corroded or damaged connector
  • Defective brake booster
  • Defective PCM

For vehicles with hydraulic brake booster:

  • Power steering malfunctions
  • Low hydraulic brake booster pressure
  • Low power steering fluid
  • Problems with the power steering pump

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, as the voltage requirement is usually based on the make, model, and specific year of the vehicle.

Circuit Checks

As said earlier, the voltage requirement may vary between vehicles, 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 faulty brake booster pressure sensor
  • Cleaning corrosion from connectors
  • Repair or replacement of faulty wiring
  • Replacement of brake booster
  • Repair or replacement of power steering pump, fluid connection, fluid and lines level
  • Replacement of PCM (rare)

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.