P0555: Brake booster vacuum sensor signal outside its electrical range

P0555 concerns the brake booster pressure-sensing circuit. Mazda's SKYACTIV-G procedure calls the component the power brake unit vacuum sensor and monitors its signal whenever the ignition is on.

What the monitor detects

The documented PCM sets this fault when the signal stays below 0.15 V or above 4.8 V for five seconds. Either electrical extreme can produce the same code. This makes the actual stored voltage important: P0555 alone does not establish low brake vacuum or identify which wire has failed. The listed response is inhibition of i-stop engine stopping.

How the evidence directs diagnosis

Mazda checks the sensor connector, its supply and signal circuits for grounding faults, and the PCM connector before assessing shorts to power, cross-circuit shorts and opens. Supply, signal and ground integrity all matter; replacing the vacuum sensor cannot correct a damaged supply wire. The sensor itself is evaluated after those circuit checks.

Because this measurement belongs to brake assistance, inspection and any brake-unit work require the vehicle-specific professional procedure. An electrical code is not evidence that braking performance is acceptable. Repair confirmation includes the specified key-on or running self-test, a check for recurrence as a pending fault, and review of other DTCs. Preserve the original signal and operating-state evidence before clearing memory.

Documented application

Mazda3 workshop manual 01/2019 onward, d9n1-1a-22d_ver10; PCM (SKYACTIV-G). Verify exact vehicle specification; this is a Mazda application example. Mazda factory diagnostic page, mirrored by CHARM.