P0557: Brake Booster Pressure Sensor Circuit Low Input

What it means

Same as P0555 with the signal voltage below the plausible range. Boost control tracks commanded vs. actual manifold pressure; persistent deviation means wastegate, vanes, hose or actuator are not delivering the requested pressure.

At a glance

• Usually lights the check-engine light

How it works

The brake-booster pressure sensor sends a signal to the control module so it can assess pressure available for brake assist. P0557 sets when that signal falls below the calibrated electrical range, which points to a low-input circuit fault and does not by itself prove that booster pressure is low.

Common causes

Most likely first.

1. Signal shorted to ground
2. Connector pins corroded - high contact resistance
3. Harness chafing through insulation

Symptoms

• Check-engine light on
• Stop-start or brake-assist features may be disabled

How to diagnose it

1. Save freeze-frame data and check live brake-booster pressure and related warnings to see whether the low signal is present continuously or under particular conditions.
2. Inspect the sensor connector and harness for corrosion, loose pins, crushed sections, or insulation rubbed through to ground.
3. Check the sensor signal circuit for a short to ground and test its continuity against the manufacturer's specification.
4. Verify the sensor power and ground connections and compare circuit readings with the manufacturer's specification.
5. If wiring and connections pass, test the brake-booster pressure sensor against the manufacturer's specification.

Common fixes

• Clean and secure corroded or loose connector pins.
• Repair wiring chafed through to ground.
• Repair an open or damaged sensor circuit.
• Replace the pressure sensor if it fails testing.

Can I keep driving?

If the brake pedal feels normal, drive cautiously to a repair location, keeping in mind that brake-assist features may be disabled. Stop and seek assistance if the pedal becomes hard or braking effort increases.