What Does P0557 Mean?
The ECM has detected that the brake booster pressure sensor circuit is producing a voltage below the minimum acceptable threshold. This low-input condition indicates the sensor signal is being pulled toward ground, either through a wiring short, failed sensor, or lost reference voltage. The ECM interprets this as maximum vacuum or an impossibly low pressure reading that does not match real conditions.
Safety Warning
A stuck-low brake booster pressure signal causes the ECM to believe maximum vacuum is always present, preventing it from detecting a real loss of brake assist. This could mask a dangerous brake booster failure condition.
Common Causes
35%
Brake booster pressure sensor signal wire shorted to ground due to chafed wiring or moisture in the connector
30%
Failed brake booster pressure sensor with internal short producing abnormally low voltage
20%
Lost 5V reference voltage to the brake booster pressure sensor from a shorted sensor on a shared reference bus
15%
Corroded sensor ground connection creating a voltage divider that pulls the signal low
Diagnostic Steps
1
Step 1: Disconnect the brake booster pressure sensor. With key on, check if the ECM signal voltage goes to the expected open-circuit value (near 5V). If it does, the sensor is internally shorted.
2
Step 2: Verify the 5V reference voltage is present at the sensor connector with the sensor disconnected. If reference voltage is missing or low, check for a shorted sensor on the shared reference bus.
3
Step 3: Measure resistance from the signal wire to ground at both the sensor connector and ECM connector. Any reading below 10k ohms indicates a short to ground in the harness.
4
Step 4: Inspect the wiring harness from the sensor to the ECM for chafed insulation, pinched wires, or moisture damage. Pay attention to areas near the brake master cylinder where brake fluid may have leaked onto the harness.
5
Step 5: Replace the sensor if it is internally shorted. Repair any wiring shorts. Clear codes and verify the sensor reads within the expected range at idle (high vacuum) and with the engine off after multiple brake applications (low vacuum).
Estimated Repair Cost
$60 - $300
Parts + labor, varies by vehicle and location
The ECM has detected that the brake booster pressure sensor circuit is producing a voltage below the minimum acceptable threshold. This low-input condition indicates the sensor signal is being pulled toward ground, either through a wiring short, failed sensor, or lost reference voltage. The ECM inte...
The most common cause of P0557 (Brake Booster Pressure Sensor Circuit Low Input) is: Brake booster pressure sensor signal wire shorted to ground due to chafed wiring or moisture in the connector
Typical repair costs for P0557 range from $60 to $300, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A stuck-low brake booster pressure signal causes the ECM to believe maximum vacuum is always present, preventing it from detecting a real loss of brake assist. This could mask a dangerous brake booster failure condition.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0557 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
OBD2 Scanner
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