P0557Generic code

P0557 Code: Brake Booster Pressure Sensor Circuit Low

The P0557 code means the brake booster pressure sensor signal is below its minimum value. The main risk is a possible loss of brake assist. It usually comes from the signal wire shorted to ground or from a missing reference voltage.

Severity
High
Can I keep driving?
Yes, but with caution
Estimated cost
$40 to $300
Difficulty
Medium
System
Chassis, Brakes and Suspension
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0557 code means

On vehicles with an electric vacuum pump, or with engines that make little vacuum (direct injection, turbo, hybrids), a sensor measures the brake booster vacuum so the pump can be turned on or the engine adjusted.

The sensor gets a 5 V reference and a ground from the computer and returns a signal between about 0.5 and 4.5 V. A low signal, which is what the P0557 code reports, is usually caused by the signal wire shorting to ground or by a missing reference.

Without this signal, the computer does not know whether there is enough brake assist. It may run the vacuum pump continuously or turn on a brake warning. Always check that the brake pedal has normal assist.

To confirm it, unplug the component with the key on and watch what happens to the reading or the code. If the value jumps to the opposite extreme, the wiring is fine and the component is the one pulling the signal to ground. If nothing changes, look for the short in the wire, starting where the harness rubs against the engine, the chassis or the exhaust.

P0557 code symptoms

  • Check engine light on.
  • Possible loss of brake assist.
  • Hard brake pedal.
  • Vacuum pump running continuously.
  • Brake warning.

Likely causes of the P0557 code

  1. Signal wire shorted to ground. The harness rubs against the engine, the chassis or a bracket.
  2. Reference supply shorted. If several sensors fail at once, suspect the shared reference.
  3. Damaged sensor with an internal short.
  4. Connector with water or corrosion. It creates a path to ground.

How to diagnose the P0557 code step by step

  1. Read the sensor value on the scan tool and compare it with what you expect.
  2. Inspect the connector for corrosion, water or loose pins.
  3. Check the supply and ground with the key on: the reference should be correct, with less than 0.1 V of voltage drop on the ground.
  4. Measure the signal with the connector plugged in and watch whether it changes with the condition.
  5. Do a wiggle test on the harness with the scan tool in graph mode.
  6. Check the continuity of each wire to the computer (under 1 ohm).

How to fix the P0557 code

  • Repair the connector or the wiring.
  • Replace the sensor if the tests confirm it is bad.
  • Correct the real condition if there is one.
  • Clear the codes and verify with a road test.

Technical reference data

  • 5 V reference, ground and a signal proportional to the vacuum.
  • These values are a general guide: check the manufacturer data for your vehicle.

Brand-specific notes

This code is usual on vehicles that rely on an electric vacuum pump or that make little engine vacuum.

P0557 on BMW

Usual on BMW vehicles.

P0557 on Volkswagen

Usual on Volkswagen vehicles.

P0557 on Ford EcoBoost

Usual on Ford EcoBoost engines.

P0557 on Hybrids

Usual on hybrid vehicles.

Frequently asked questions about P0557

Can I drive with a P0557 code?
With great caution. If the brake pedal is hard, stopping takes more force, so have it checked as soon as you can.
What does the P0557 code mean?
It is a generic OBD2 code that means the brake booster pressure sensor signal is below its minimum value. In practice that points to a signal wire shorted to ground or a missing reference.
Do I have to replace the sensor?
Only if the tests confirm it. The connector and the wiring fail just as often.
What does the brake booster pressure sensor do?
On vehicles with an electric vacuum pump, or with engines that make little vacuum (direct injection, turbo, hybrids), a sensor measures the brake booster vacuum so the pump can be turned on or the engine adjusted.
How much does it cost to fix P0557?
As a rough guide, between $40 and $300. A wiring or connector repair is at the low end, and a new sensor with labor is at the high end.
Will a P0557 code clear itself?
Only if the short was temporary, for example water in the connector that dried out. A wire rubbing on metal will bring the P0557 code back.
What is the most common cause of P0557?
A signal wire shorted to ground, usually where the harness rubs against the engine, the chassis or a bracket. A short in the shared reference supply and a damaged sensor follow.

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