P0556Generic code

P0556 Code: Brake Booster Pressure Sensor Circuit Range/Performance

The P0556 code means the brake booster pressure sensor gives a reading that is electrically valid but does not match what the computer expects. The main risk is a possible loss of brake assist. The sensor, the connector and the wiring fail about equally often, so test before you buy parts.

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 P0556 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 range or performance fault, which is what the P0556 code reports, means the sensor is reading but reading wrong, or that there is a real abnormal condition.

Without a believable 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.

This kind of fault rarely shows up with a multimeter, because everything looks electrically correct. You have to compare what the part says against reality: with a gauge, a thermometer, a tachometer, or by commanding the component with the scan tool and seeing whether the physical result matches the command.

P0556 code symptoms

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

Likely causes of the P0556 code

  1. Degraded or out-of-calibration sensor. It gives believable readings that are wrong.
  2. A real abnormal condition. The system really is running outside normal.
  3. High resistance in the connector. Corrosion shifts the reading.
  4. Damaged or blocked port, hose or reluctor wheel, depending on the type of sensor.

How to diagnose the P0556 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 P0556 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.

P0556 on BMW

Usual on BMW vehicles.

P0556 on Volkswagen

Usual on Volkswagen vehicles.

P0556 on Ford EcoBoost

Usual on Ford EcoBoost engines.

P0556 on Hybrids

Usual on hybrid vehicles.

Frequently asked questions about P0556

Can I drive with a P0556 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 P0556 code mean?
It is a generic OBD2 code that means the brake booster pressure sensor gives a reading that is electrically valid but does not match what the computer expects. The result can be a loss of brake assist.
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 P0556?
As a rough guide, between $40 and $300. A connector repair is at the low end, and a new sensor with labor is at the high end.
Will a P0556 code clear itself?
Not reliably. If the sensor is drifting or a connector is corroded, the P0556 code comes back, so fix the cause instead of just clearing it.
What is the most common cause of P0556?
A degraded sensor or a corroded connector that shifts the reading. Because the sensor and the connection fail about equally often, test both before you buy a part.

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