What Does P0556 Mean?
The ECM has detected that the brake booster pressure sensor signal is not changing as expected for current operating conditions, or its readings are outside the acceptable range/performance window. The sensor may be reading but its values do not correlate with expected brake booster pressure based on engine vacuum and brake pedal activity. This indicates sensor inaccuracy or degraded response.
Safety Warning
Inaccurate brake booster pressure readings can prevent the ECM from detecting low vacuum conditions that reduce braking assist. The driver may not be warned of degraded brake booster performance, potentially resulting in longer stopping distances.
Common Causes
30%
Degraded brake booster pressure sensor with calibration drift or sluggish response to pressure changes
30%
Brake booster vacuum leak causing actual pressure to fall outside the ECM's expected range for current conditions
25%
Intermittent connection at the brake booster pressure sensor connector causing inconsistent readings
15%
Restricted or leaking brake booster vacuum supply preventing proper pressure buildup
Diagnostic Steps
1
Step 1: Inspect the brake booster vacuum hose from the intake manifold to the booster for cracks, collapse, or disconnection. Check the one-way check valve by removing it and verifying it only allows airflow toward the booster.
2
Step 2: Use a vacuum gauge at the brake booster to measure actual vacuum. With the engine running, vacuum should be 18-22 inHg. Compare this reading to what the ECM reports via the brake booster pressure sensor.
3
Step 3: Pump the brake pedal 5-6 times with the engine off to deplete booster vacuum, then start the engine. Monitor the sensor live data; it should show vacuum rebuilding within 2-3 seconds. A slow or absent response indicates a sensor or vacuum supply issue.
4
Step 4: Wiggle-test the sensor connector while monitoring live data to check for intermittent contact. Check the connector for moisture, corrosion, or loose pins.
5
Step 5: If vacuum supply is verified good and the sensor reading does not match actual conditions, replace the brake booster pressure sensor. Clear codes and verify proper response through a series of engine-off brake applications followed by engine restart.
Estimated Repair Cost
$60 - $400
Parts + labor, varies by vehicle and location
The ECM has detected that the brake booster pressure sensor signal is not changing as expected for current operating conditions, or its readings are outside the acceptable range/performance window. The sensor may be reading but its values do not correlate with expected brake booster pressure based o...
The most common cause of P0556 (Brake Booster Pressure Sensor Circuit Range/Performance) is: Degraded brake booster pressure sensor with calibration drift or sluggish response to pressure changes
Typical repair costs for P0556 range from $60 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Inaccurate brake booster pressure readings can prevent the ECM from detecting low vacuum conditions that reduce braking assist. The driver may not be warned of degraded brake booster performance, potentially resulting in longer stopping distances.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0556 to identify the root cause.
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Quick Info
Category
Powertrain
System
Brake Assist / Brake Booster System
Difficulty
Type
Generic (SAE)
Recommended Tools
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