OBD-II Code P0557 — What It Means & How to Fix It

What Does This Error Mean?

P0557 means the brake booster pressure sensor is sending a voltage signal that is lower than the PCM expects. This usually means the sensor circuit is shorted to ground, the sensor has failed, or the connector is damaged. Since this involves the brake boost system, it is a safety-related fault. Check your brake pedal feel right away — if braking seems harder than normal, stop driving and get the vehicle inspected.

Affected Models

2005 and newer cars and trucks with monitored brake booster systems
All Toyota and Honda hybrid models with electric vacuum pumps
Ford and GM trucks with vacuum-boosted brakes and pressure sensors
Chrysler, Dodge, and Jeep vehicles with booster pressure monitoring
European models with electrohydraulic brake boost systems (BMW, Mercedes, Audi)

Common Causes

Brake booster pressure sensor has shorted internally — signal wire reads low or zero voltage
Sensor signal wire is shorted to ground — damaged insulation in the wiring harness near the booster
Corroded or damaged connector at the brake booster pressure sensor with a pin-to-pin short
Sensor reference voltage supply wire is open — sensor has no power and reads low by default
PCM input circuit fault causing the brake booster sensor signal to appear lower than it actually is

How to Fix It

Assess your braking performance first. Gently press the brake pedal with the engine running. Normal pedal feel means the booster is likely still functioning. Hard pedal, increased effort, or longer stopping distances require immediate attention — do not drive further.

The sensor circuit fault does not always mean the booster itself has failed, but it is a warning sign that demands prompt diagnosis.
Locate the brake booster pressure sensor on the booster body (usually on the driver's side firewall). Inspect the wiring connector for corrosion, bent pins, or damage. Unplug and reseat the connector — sometimes a loose connection is the entire problem.

Spray electrical contact cleaner into the connector if corrosion is visible. Dry completely before reconnecting.
With the sensor disconnected, probe the sensor signal wire at the harness connector and measure voltage to ground. With the ignition on, you should see the reference voltage (typically 5 volts). Zero volts means the reference supply wire or the PCM output has failed.

If reference voltage is present at the connector but the sensor still reads low when plugged in, the sensor itself is shorted internally.
Check the sensor signal wire for a short to ground. Disconnect the sensor connector and use a multimeter in continuity mode to test between the signal pin and chassis ground. Continuity here means the wire is shorted to ground somewhere in the harness.

Trace the harness from the sensor toward the firewall and look for pinched or chafed wiring.
Replace the brake booster pressure sensor if the wiring checks out good. The sensor is threaded into the booster housing on most vehicles. Disconnect the connector, remove the sensor, install the new unit to the correct torque, reconnect, and clear the code.

Confirm the new sensor reads correctly using live data on a scan tool before considering the repair done.

When to Call a Professional

Brake system codes are safety-critical — please have this inspected soon. If your pedal feels normal, you can likely drive short distances to reach a shop. If the pedal feels hard or your stopping distance has increased, do not drive — call for roadside assistance. Repair cost is typically $150 to $500 depending on the fault.

Frequently Asked Questions

Is P0557 more serious than other check engine light codes?

Yes — brake-related codes are always higher priority than engine management codes like misfire or oxygen sensor faults. While many check engine codes simply affect fuel economy or emissions, a brake booster fault can affect your ability to stop safely. Address P0557 before any other pending codes.

Can P0557 come on from a bad electrical connection and not an actual booster problem?

Absolutely — a corroded connector at the sensor is one of the most common causes of P0557. Clean the connector, reseat it, and clear the code to see if it returns. If the code comes back quickly, move on to testing the sensor and wiring.

Does the 5-volt reference voltage at the sensor connector always indicate the PCM is working?

Not necessarily — the 5V reference supply comes from the PCM, but the PCM may have an internal fault affecting only that circuit. If you have confirmed correct voltage at the connector but the PCM still reads the sensor low, suspect either a faulty sensor or a PCM input circuit fault.