Неисправный датчик давления вакуумного усилителя

Monitors the vacuum or pressure available to assist the brakes, so the vehicle knows whether the driver's effort is being multiplied as it should be.

Its importance comes from what happens without assistance: the brakes still work, and they need several times the pedal effort. A driver who has never felt an unassisted pedal will often conclude the brakes have failed when they have not — which is precisely why the vehicle watches for it and warns.

On modern engines the assistance is less abundant than it used to be. A throttled petrol engine once produced plentiful manifold vacuum; direct injection, turbocharging and stop-start have all reduced it, so many vehicles now carry an electric vacuum pump. That pump is a leading cause of trouble here — it is a wear item, and a failing one leaves pressure the sensor correctly reports as insufficient.

So the first question is whether the sensor is wrong or the vacuum genuinely is, and they lead to completely different repairs. The clue is available to the driver: a pedal that has become noticeably harder, or that goes hard after two or three presses with the engine off, is a real vacuum problem rather than a sensor reporting one.

A perished booster hose or its one-way valve is the cheap end of that, and worth checking before anything else.