P0557
Brake Booster Pressure Sensor Circuit Low Input
Powertrain Chassis/Safety Brake system sensors 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

Your car's brake booster pressure sensor is sending a signal that's too low, like a battery that's losing its charge. The ECU thinks there's a vacuum leak or sensor malfunction affecting your braking system's power assistance.

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Symptoms You May Notice
3 known symptoms for this code
Hard brake pedal requiring excessive force to stop
Check Engine Light illuminated on dashboard
Possible brake warning light or reduced braking performance
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECU monitors the brake booster vacuum pressure sensor voltage output, which should increase as vacuum pressure increases. When the sensor voltage falls below the minimum threshold (typically 0.5V), the ECU logs a P0557 fault. This indicates either a weak signal from the sensor, a vacuum leak, or a circuit problem.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Sensor voltage output 0.5V - 4.5V Below 0.5V (Low Input)
Booster vacuum pressure 15-22 inHg at idle Below threshold or no signal
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Vacuum hose to booster
Inspect for cracks, splits, or disconnection; reconnect or replace if damaged.
2
Brake booster pressure sensor connector
Clean corroded pins and reseat the connector firmly to restore electrical contact.
3
Brake booster pressure sensor
Replace if voltage remains low after hose and connector checks confirm good vacuum pressure.