P0557

Universal

Brake Booster Pressure Sensor Circuit Low Input

Definition source: General OBD-II reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Medium
Medium Severity

The brake booster pressure sensor reports pressure information to a vehicle control system so it can monitor the booster. This code means the sensor circuit is reporting a low electrical input. It does not by itself prove that booster pressure is low or that the sensor has failed. Depending on the cause, you may notice a warning light, no change in how the vehicle drives, or reduced brake assist.

Report an error with this code

⚠️ Before You Begin (Safety First)

If the brake pedal becomes unusually hard, braking feels reduced, or a brake warning light appears, pull over safely and arrange assistance rather than continuing to drive. Reduced brake assist can require much more pedal effort. If braking feels normal, have the fault diagnosed promptly and watch for any change.

Common Symptoms

  • There may be no noticeable symptoms if the circuit fault does not affect brake assist.
  • A brake or system warning light may appear if the vehicle detects the fault.
  • If booster operation or pressure is actually affected, the brake pedal may feel harder and require more effort.
  • Braking may feel different or less assisted if the booster cannot provide normal assist; the code alone does not establish that this has happened.

Common Causes

  • A damaged wire or short to ground may pull the sensor signal low.
  • A loose, corroded, wet, or damaged sensor connector may interfere with the signal.
  • The pressure sensor may have an internal fault.
  • A problem with the sensor's power or ground circuit may cause an incorrect low input.
  • A booster pressure or supply problem may affect the sensor reading, depending on the vehicle's system.

Diagnostic Steps

  1. Note any warning lights and whether the brake pedal feels different. If braking feels impaired, stop driving and arrange assistance.
  2. With the vehicle safely parked and switched off, visually check accessible wiring and the sensor connector for obvious damage, looseness, or corrosion. Do not unplug components unless the vehicle-specific procedure says it is safe.
  3. If you have a suitable scan tool, record this code and any related codes before clearing anything; related faults can help narrow the diagnosis.
  4. Look for obvious damage to accessible booster hoses only if you can identify them safely. Do not disconnect hoses or dismantle brake components as a diagnostic experiment.
  5. If the fault remains or the brake feel is abnormal, have a qualified technician test the circuit and booster using vehicle-specific service information.

When to Call a Professional

A technician can check for related codes and interpret sensor data, then use the vehicle-specific wiring diagram to test the circuit, connector, power and ground as applicable. They can also verify booster operation and pressure with suitable test equipment. The code alone is not a reason to replace the sensor.

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Commonly Asked Questions

Does P0557 prove the brake booster pressure sensor is faulty?

No. It identifies a low electrical input in the sensor circuit. Wiring, the connector, power or ground, the sensor, or a pressure-related issue may be involved.

Does “low input” mean the booster pressure itself is low?

Not necessarily. The code describes the circuit input, which can be low because of an electrical fault. A technician must check the sensor signal and booster operation to distinguish the possibilities.

Can I keep driving with this code?

The code alone does not show whether brake assist is affected. If the pedal feels unusually hard, braking feels reduced, or a brake warning light appears, stop safely and arrange assistance. Otherwise, get the fault checked promptly.

Should I replace the sensor first?

Not without testing. Checking the connector and wiring, circuit power and ground as applicable, sensor data, and booster operation can prevent replacing a part that is not the cause.

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