Powertrain, generic · Vehicle speed, idle control and auxiliary inputs

P0556 Brake Booster Pressure Sensor Circuit Range/Performance

Check engine lightOn
EmissionsNot relevant
Drive cycleRequired
Limp modeNo

The brake booster pressure sensor (5 V reference, signal, ground) measures the vacuum in the brake booster. On direct-injection, turbo, stop-start and hybrid engines intake vacuum is scarce, so the engine module watches it and creates vacuum when needed – closing the throttle, suppressing engine stop or running an electric vacuum pump. That makes it an input to idle and throttle control. P0556 means the signal is valid but implausible. GM compares it with intake vacuum (MAP) with the throttle nearly closed for over 3 s, filtered, type B with two trips. VW and Porsche also check the electric vacuum pump with it – Porsche compares the pressure at wake-up and 1.5 s later – so P0556 can also mean a weak pump or a leak. VW fixed false setting on the 2016–17 Jetta 1.4 T by software.

Likely causes

Leaking or kinked vacuum line or faulty check valve
Sensor stuck or drifting
Electric vacuum pump or its relay weak (hybrid, Porsche, turbo DI)
Module software (false setting)
Booster diaphragm leaking

Symptoms

  • Check engine light and/or brake booster warning
  • Stop-start not working
  • Hard brake pedal with real vacuum loss

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–400
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

brake booster pressure sensorvacuum lineelectric vacuum pumpengine control module

Related codes

P0555P0557P0558P0559P0106P050F

References: gsi.ext.gm.com, gsi.ext.gm.com, static.nhtsa.gov, static.nhtsa.gov.