Will replacing the sensor fix this?

Usually not, and that is what separates this code from the other four in the family. Range and performance means the signal is electrically healthy but does not agree with the rest of the picture, which is exactly what happens when a perfectly good sensor reports a genuinely broken vacuum system. Check the booster, the check valve and the hoses with a hand vacuum pump first. The sensor becomes a fair suspect only when the vacuum side tests good and the reading still refuses to move with the pedal.

My pedal gets hard after a couple of pumps with the engine off. Is that related?

Almost certainly, and it is the most useful symptom you can bring to a diagnosis. A healthy booster stores enough vacuum for two or three assisted applications after shutdown. Losing assist immediately means the vacuum is not being held, which is either a torn diaphragm or a check valve that no longer seals. Both produce this code honestly. It is worth attending to promptly, because the same fault means less assist available in a hard stop when the engine is under load and making little vacuum.

Why is my idle rough at the same time?

Because a booster leak is an intake leak. The booster draws from the intake manifold, so a torn diaphragm or a split supply hose lets air into the engine that never passed the mass airflow sensor. The engine runs lean, the idle wanders, and you may see fuel trim or lean codes stored alongside this one. That combination is actually helpful — a rough idle plus a booster code points strongly at a mechanical vacuum leak rather than at anything electrical.

The check valve is cheap. Should I just change it first?

It is a reasonable first move on this particular code, provided you test it rather than replacing it blind. Pull it out, blow through it both ways and confirm it passes freely one way and seals completely the other. If it fails that test, it is a genuinely cheap fix for an expensive-sounding fault. If it passes, you have spent five minutes and eliminated the least expensive possibility, which makes the next step — testing whether the booster itself holds vacuum — much more meaningful.

OBD-II trouble code

P0556: Brake Booster Pressure Sensor Circuit Range/Performance

The booster vacuum signal is present and electrically healthy but does not match what the engine is doing. This is the one code in the family that an entirely good sensor sets, and the repair is usually mechanical.

Medium severityPowertrainBrakes / Vacuum SupplyDrivable short-term

Quick facts

System: Powertrain
Category: Brakes / Vacuum Supply
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $60 – $1,100
DIY difficulty: Intermediate DIY

What does P0556 mean?

Range and performance codes are correlation checks, and they behave differently from every other code in their family. The module is not measuring the circuit here — it is comparing this sensor's output against what it already knows from elsewhere, principally manifold pressure, engine speed and brake pedal position, and deciding whether the number is believable. When it is not, P0556 is stored.

That has an important consequence: a sensor doing its job perfectly will set this code if the thing it is measuring is genuinely wrong. Vacuum that never reaches a normal level, vacuum that bleeds away instead of holding, vacuum that does not drop when the pedal is applied — all of those are honest reports of a broken vacuum system. Replacing the sensor in that situation changes nothing except the bill, and it is the most common wasted repair on this code.

So the productive question is which direction the plausibility check failed in, and there are two.

The first is vacuum that never builds. The booster is supposed to be pulled down to something near manifold vacuum and held there by a one-way check valve. If the supply hose has softened and collapsed, if the check valve has failed, or if an auxiliary pump has lost its ability to pull, the level never reaches what the module expects at that engine speed. Anything that goes hard, splits or cracks in the vacuum path produces this, and hoses in the hot area behind the engine do all three with age.

The second is vacuum that never changes. Applying the brakes admits atmospheric air to the rear chamber of the booster, and the sensor should see a distinct dip followed by recovery. A reading that sits at a constant value through repeated firm applications is not reporting on anything — the port feeding the sensor may be blocked with debris, or the sensing element may have gone lazy and stopped responding to real changes. This is the direction in which the sensor itself is genuinely the likely fault.

A torn booster diaphragm sits underneath both patterns and deserves a mention of its own, because it is the failure with the largest consequence. A booster that will not hold vacuum gives a pedal that goes hard after one or two applications with the engine off, and the same fault that stores this code also lets unmetered air into the intake, which is why a rough idle and a hiss from behind the pedal often arrive at the same time as the light.

Common causes

Torn or perforated booster diaphragm that cannot hold vacuum
Failed one-way check valve letting stored vacuum bleed back toward the manifold
Collapsed, split or hardened vacuum supply hose between the source and the booster
Weak or failing auxiliary vacuum pump that runs but cannot pull the booster down
Blocked or restricted sensing port, leaving the sensor reading a trapped, unchanging pressure
Lazy sensing element that responds far too slowly to real vacuum changes
Cracked plastic elbow or fitting at either end of the vacuum line
Vacuum being stolen by another leak in a shared line, so the booster never gets its share

Symptoms

Brake pedal that becomes noticeably harder after one or two applications with the engine off
Hissing from behind the pedal when the brakes are applied
Rough or unstable idle caused by the same leak that is starving the booster
Auxiliary vacuum pump running far more often than normal or almost continuously
Stop-start refusing to shut the engine off, or restarting it early after a stop
Longer pedal travel and increased effort needed for the same amount of braking
Booster vacuum value that stays flat on live data while the pedal is worked
Engine stumbling briefly at the moment the brake pedal is first pressed

Diagnostic steps

1.Resist the urge to reach for a meter. This code is set by a signal that is electrically fine, so the useful first tool is a hand vacuum pump. Pull the check valve out of the booster, apply vacuum to the booster port and watch whether it holds. A booster that bleeds down in seconds has a torn diaphragm and explains the code entirely.
2.Test the check valve on its own next. It should pass air freely in one direction and seal completely in the other. A valve that leaks backwards lets the booster lose its stored vacuum every time manifold pressure rises, which produces a reading that keeps falling when the module expects it to hold.
3.Determine which direction the plausibility failed. Vacuum that never builds to a normal level points at supply: a collapsed hose, a failed check valve, a weak auxiliary pump. Vacuum that reads normal but never changes when the pedal is pressed points at the sensor's port being blocked or the sensor being lazy. Those are opposite repairs and this is the split that matters.
4.On vehicles with an electric vacuum pump, confirm the pump runs and produces meaningful vacuum. Command it with a scan tool if the calibration allows, or listen for it after shutdown. A pump that runs but cannot pull the booster down is a mechanically failed pump reporting through this code.
5.Inspect every vacuum connection between the source and the booster with the engine idling. Hoses in this area go hard and split at the ends, and hard plastic elbows crack where they are pushed on. A leak here shows up as a reading that never reaches the expected value even though everything electrical checks out.
6.Compare the booster reading against manifold absolute pressure at the same moment. On a naturally aspirated engine at idle they should track together fairly closely, and a wide, persistent gap between them proves the loss is between the manifold and the booster rather than inside the sensor.

Repair cost

$60 – $1,100

This is by far the widest range in the family and that is honest rather than hedging, because the fault is mechanical and the failed parts differ enormously in price. A split vacuum hose or a failed check valve is $60 to $180 fitted and is a genuinely common outcome. A replacement sensor, if the port or the element really is the problem, is $150 to $400. An electric vacuum pump is $250 to $700. A complete brake booster is the expensive end at $400 to $1,100 fitted, because the master cylinder has to come off and the system has to be bled. Test the vacuum side mechanically before authorising any parts, since the cheap repair and the expensive one produce the same code.

Estimate your repair

Run the numbers for your vehicle

Open the Repair Cost Estimator with brake booster / vacuum supply service preselected. Adjust labor rate and vehicle category to fit your situation.

Estimate your repair cost

DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.