OBD-II Code P0556 — What It Means & How to Fix It

What Does This Error Mean?

P0556 means the brake booster pressure sensor is operating but its readings are outside the expected range or not changing correctly under different conditions. Unlike P0555 which indicates a circuit fault, P0556 means the sensor is working but giving implausible data. This typically points to a slow vacuum leak, a failing booster, or a sensor that is not responding as it should. Since this involves your brake system, treat it as a priority repair.

Affected Models

2005 and newer vehicles with electronically monitored brake booster vacuum — Ford, GM, Chrysler, Toyota, Honda
All hybrid vehicles with electric vacuum pumps and booster pressure sensors
Diesel trucks that use engine vacuum for brake boost
High-performance vehicles with sport brake packages that monitor booster pressure
Any vehicle with a PCM-monitored brake booster pressure sensor

Common Causes

Slow vacuum leak at the booster hose, check valve, or booster diaphragm — pressure drops faster than expected
Brake booster diaphragm is cracking or worn — vacuum is lost through the diaphragm instead of being held
Brake booster check valve is sluggish or partially blocked — vacuum builds slowly and falls out of the expected range
Brake booster pressure sensor is giving a lazy or incorrect response — correct voltage range but wrong reaction timing
On hybrid vehicles, the electric vacuum pump is weak and cannot maintain booster pressure within specification

How to Fix It

Test brake pedal firmness before anything else. With the engine running, press the pedal firmly and hold it for 30 seconds. The pedal should remain firm with no sinking. A pedal that slowly sinks indicates a booster or master cylinder issue.

If the pedal sinks toward the floor while held, the problem is likely a leaking brake booster diaphragm or master cylinder. Get the vehicle to a shop immediately.
Inspect the vacuum hose and fittings at the brake booster. Listen for a hissing sound with the engine running — a hiss near the booster indicates a vacuum leak. Feel around the hose connections for air movement.

Even a small vacuum leak can cause P0556 because the PCM expects the booster to hold vacuum for a defined period after the engine is shut off.
Test the booster check valve — remove it and blow through both directions. The valve should only allow airflow from the booster toward the engine. A leaking valve allows pressure to equalize and will cause erratic sensor readings.

A check valve costs under $20 and is the most common cause of booster vacuum loss. Replace it first before condemning the booster.
Use a scan tool to monitor the brake booster pressure sensor reading in real time. The value should change predictably when the brake pedal is pressed and released. A sensor that shows a flat line or erratic jumps while the booster vacuum changes may need replacement.

Compare the reading when the engine is running versus off — the reading should hold steady for several minutes after shutdown if the check valve and booster are healthy.
If all physical components check out, replace the brake booster pressure sensor. The sensor is usually mounted directly on the booster body. Disconnect the connector, unscrew the sensor, install the new one, reconnect, and clear the code.

After replacement, verify the code does not return over several drive cycles before considering the repair complete.

When to Call a Professional

Brake booster failure is a safety-critical repair. If you notice increased pedal effort or longer stopping distances, do not drive the vehicle. A qualified brake technician can diagnose whether the booster, check valve, or sensor is at fault. Booster replacement typically costs $300 to $700 at a shop including labor.

Frequently Asked Questions

What is the difference between P0555 and P0556?

P0555 means a hard circuit fault — the sensor signal is completely missing or outside the electrical operating range. P0556 means a performance or rationality fault — the sensor is electrically functioning but the readings do not match what the PCM expects under the current conditions. P0556 is often caused by mechanical issues like a slow vacuum leak rather than electrical problems.

Can a bad vacuum pump cause P0556 on a hybrid?

Yes — on hybrid vehicles the engine does not always run, so an electric vacuum pump maintains booster pressure. If the pump is weak or failing, it cannot keep up with booster demand and pressure drops below the acceptable range. The pump is a separate item from the booster and can be tested and replaced independently.

Will P0556 cause my car to fail an emissions test?

Likely yes — P0556 stores a fault code that typically sets the readiness monitors to not ready. Most states require all monitors to be ready for an emissions test. Fix the brake booster issue first, then drive normally for a few days to allow monitors to reset before testing.