P0556 moderate

Brake Booster Pressure Sensor Circuit Range/Performance

The P0556 code means the vehicle's onboard diagnostic system has detected: Brake Booster Pressure Sensor Circuit Range/Performance. This is a moderate severity code.

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Keep driving?: Short distances only
DIY difficulty: moderate
Estimated cost: $100 - $500 (parts and labor)

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Common Symptoms

Check Engine Light on
Brake feel inconsistent
Start/stop system disabled
Idle speed fluctuations when braking

Probable Causes (Ranked by Likelihood)

Brake booster pressure sensor out of calibration Very High
Vacuum leak in brake booster system High
Brake booster diaphragm leaking internally Moderate
Intermittent sensor connection Moderate

Step-by-Step Diagnostic Procedure

Compare brake booster sensor reading to a known good vacuum gauge connected to the booster line
Check all vacuum connections at the brake booster for leaks
Listen for a hissing sound at the brake booster when the brake pedal is pressed, indicating an internal diaphragm leak
Monitor sensor reading during rapid brake pedal pumping to check for response lag

Frequently Asked Questions

What does range/performance mean for P0556?

Range/performance means the brake booster pressure sensor is providing a reading but it does not match what the ECM expects based on engine vacuum conditions. The sensor reading may be too high, too low, or slow to respond.

Can a leaking brake booster cause P0556?

Yes. A brake booster with an internal diaphragm leak will not maintain proper vacuum, causing the sensor reading to deviate from expected values. This is a safety concern as it also reduces brake assist power.

How do I test the brake booster?

With the engine off, pump the brake pedal several times to deplete vacuum. Then hold the pedal down and start the engine. The pedal should drop slightly as vacuum builds. If the pedal does not move, the booster or vacuum supply has a problem.