P0556 code meaning
Brake Booster Pressure Sensor Circuit Range/Performance
Powertrain system Generic code P05xx codes Check engine light
What P0556 means
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.
P0556 at a glance
- Check engine lightYes, usually on
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 2 hours
- Clears after repairAfter a drive cycle
Symptoms of P0556
- Check engine light and/or brake booster warning
- Stop-start not working
- Hard brake pedal with real vacuum loss
Common causes of P0556
- Most likelyLeaking or kinked vacuum line or faulty check valve
- PossibleSensor stuck or drifting
- PossibleElectric vacuum pump or its relay weak (hybrid, Porsche, turbo DI)
- Less likelyModule software (false setting)
- Less likelyBooster diaphragm leaking
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Brake booster pressure sensorVacuum lineElectric vacuum pumpEngine control module
What each part of P0556 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 5 | Area: vehicle speed, idle control and auxiliary inputs. |
| 56 | The exact fault in that area. |
What to do next
- Note P0556 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0556
What does P0556 mean?
P0556 means "Brake Booster Pressure Sensor Circuit Range/Performance". The brake booster pressure sensor (5 V reference, signal, ground) measures the vacuum in the brake booster. It is a generic code, so it means the same on every make.
Is it safe to drive with P0556?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0556 fail an emissions test?
P0556 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is P0556 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0556 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.