P0556 indicates that the brake booster pressure sensor is reporting a signal outside the expected range or is not performing within its calibrated parameters. The sensor monitors vacuum or pressure in the brake booster to help the control module manage brake assist functions.
The sensor element itself can drift or become electrically open/short, sending an out‑of‑range voltage to the control module.
Corroded, frayed, or loose wiring and connectors between the sensor and the control module can cause erratic or missing signals.
A leak in the brake booster vacuum hose, check valve, or intake manifold can alter the pressure the sensor measures, leading to a range error.
Internal diaphragm rupture or check‑valve failure changes the booster pressure characteristics, causing the sensor to read outside limits.
Rarely, an internal fault in the module’s sensor input circuit can misinterpret a valid sensor signal; this is only considered after all other causes are ruled out.
No. The control module is rarely the root cause; the sensor, wiring, vacuum system, or booster are far more likely. Replace the module only after all other possibilities have been eliminated and a module fault is confirmed by a qualified technician.
Yes. A leak that changes booster pressure can make the sensor report values outside its calibrated range, setting P0556.
The vehicle will still stop, but brake assist may be reduced, resulting in a harder pedal and longer stopping distances, especially at low speeds. Prompt diagnosis is recommended.
The code will remain stored until it is cleared with a scan tool or the control module completes a sufficient number of drive cycles without detecting the fault.