Brake Booster Pressure Sensor Circuit Range/Performance
Your brake booster pressure sensor is sending a signal that's outside the normal range, like a gauge needle stuck in the wrong zone. The engine computer can't trust the brake assist pressure reading, which affects how the brakes respond.
The ECU monitors voltage output from the brake booster pressure sensor to verify vacuum assist levels. It expects the sensor to produce a signal within a specific voltage range that corresponds to normal brake booster pressure. If the signal falls outside this range or doesn't respond correctly to engine load changes, the fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage Output | 0.5V to 4.5V (varies with booster pressure) | Below 0.5V or above 4.8V |
| Pressure Range | 0 to 25 inches of Hg vacuum | Out of range or erratic fluctuations |
Code P0556 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0556 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.