P0557 moderate

Brake Booster Pressure Sensor Circuit Low

The P0557 code means the vehicle's onboard diagnostic system has detected: Brake Booster Pressure Sensor Circuit Low. This is a moderate severity code.

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Keep driving?: Short distances only
DIY difficulty: moderate
Estimated cost: $100 - $400 (parts and labor)

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Common Symptoms

Check Engine Light on
False low vacuum reading
Start/stop system disabled
ECM may increase idle speed unnecessarily

Probable Causes (Ranked by Likelihood)

Short to ground in brake booster sensor wiring Very High
Failed sensor with internal short High
Water damage to sensor connector Moderate
ECM sensor input circuit failure Low

Step-by-Step Diagnostic Procedure

Read sensor value; an abnormally low voltage with engine running at normal vacuum confirms circuit low
Disconnect sensor and check for short to ground on signal wire
Inspect sensor connector for moisture intrusion or corrosion
Measure sensor output at different known vacuum levels using a hand pump

Frequently Asked Questions

What does circuit low mean for P0557?

Circuit low means the sensor signal voltage is below the expected minimum. The ECM interprets this as a very high vacuum reading, which may not match actual booster conditions. This is typically an electrical fault, not an actual vacuum issue.

Can P0557 cause the engine to idle high?

Yes. If the ECM reads a false low vacuum in the brake booster, it may increase idle speed to provide more vacuum to the brake system. This results in a higher-than-normal idle RPM.

Is the brake booster pressure sensor easy to replace?

On most vehicles, the sensor is mounted on the brake booster or the vacuum line and is accessible from the engine bay. Replacement typically takes 30 minutes to an hour and is a relatively straightforward repair.