Does circuit low mean I have low vacuum and weak brakes?

Not directly, and this is the trap in the code's name. On most sensors of this type the output voltage falls as vacuum rises, so a low voltage would represent very high vacuum if it were a real reading. What the code actually reports is that the voltage is below what the circuit can legitimately produce at all — an electrical fault, not a measurement. Your assist may or may not be affected depending on how the calibration handles bad data, but the code itself is not telling you the booster is empty.

The light came on after heavy rain. Is that a coincidence?

It is one of the strongest clues you can have on this code. The sensor connector sits under the cowl at the base of the windscreen, which is where water running off the glass is collected before it drains. When the drains block with leaves the tray holds water, the connector sits in it, and water bridging two terminals is a short to ground. Check the drains before you check anything electrical — and clear them properly, because otherwise the fault comes back with the next storm.

How do I tell whether the problem is the sensor or the wiring?

Unplug the sensor and watch the live value. With the connector open, the signal line should swing to the opposite rail. If it does, the wiring and the module are both fine and the fault is inside the sensor or in its ground path. If it stays pinned at the bottom with nothing plugged in, the wire is shorted to ground somewhere between there and the module, and no new sensor will change that.

Should I be worried about driving it?

Braking still works — the hydraulic system is entirely mechanical and unaffected. The concern is that some calibrations read an implausibly low value as "plenty of vacuum available" and stop running the auxiliary pump, which on a turbocharged or stop-start vehicle can leave you with less assist than you are used to during hard or repeated braking. If the pedal feels firmer than normal, treat it as urgent. If the pedal feels normal, get it fixed soon rather than immediately.

OBD-II trouble code

P0557: Brake Booster Pressure Sensor Circuit Low Input

Signal voltage below the electrical floor on the booster vacuum sensor. Read carefully before acting — on most of these sensors low voltage means high vacuum, so the instinctive interpretation of this code is backwards.

Medium severityPowertrainBrakes / Vacuum SupplyDrivable short-term

Quick facts

System: Powertrain
Category: Brakes / Vacuum Supply
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $0 – $500
DIY difficulty: Intermediate DIY

What does P0557 mean?

P0557 is stored when the module sees the booster pressure sensor's signal line sitting below the minimum voltage the circuit is ever supposed to produce, and staying there.

Before diagnosing anything, the polarity of the sensor has to be settled, because this is where the code trips people up. Most sensors of this type behave like manifold absolute pressure sensors: they output low voltage when pressure is low, which is to say when vacuum is high. So a reader who sees "circuit low" and concludes "no vacuum, no assist, dangerous" has the relationship exactly inverted on those applications. The raw electrical value being low would, if it were a real measurement, describe a vacuum stronger than the engine could possibly create.

And that impossibility is the whole point of the code. The module is not saying the vacuum is low. It is saying the voltage is at a level that no genuine physical condition can produce, which means the number is electrical in origin. Something is holding the signal line down: a short to ground, a lost reference voltage that has dragged the whole output with it, or a sensing element that has failed short internally.

Where this sensor lives makes one cause much more likely than it would be elsewhere. The booster is bolted to the bulkhead behind the engine, underneath the cowl — the plenum at the base of the windscreen where the wiper mechanism sits and where all the water running off the glass is collected and channelled to drains. When those drains block with leaves and pine needles, which they do routinely on any vehicle parked under trees, that tray holds standing water. The sensor connector sits in it. Water bridging two terminals is a short, and a short on this circuit is this code.

Washer fluid adds to the problem. It is directed into the same area, it contains alcohol and surfactants, and it leaves a conductive residue behind when it dries. A connector that has been repeatedly wetted and dried in that environment can short in dry weather from residue alone, which is why cleaning the shell properly matters more here than simply drying it out.

The practical consequence on the vehicle depends on how the calibration reads the value. Some treat an implausible reading as no information and run the fallback strategy — auxiliary pump on more often, auto-stop inhibited. Others interpret it at face value as maximum available vacuum and stop running the pump at all, which on a turbocharged or stop-start vehicle can leave genuinely less assist than the driver expects. That second behaviour is the reason this code is worth attending to promptly rather than filing under electrical nuisance.

Common causes

Signal wire shorted to ground, most often where the harness crosses the bulkhead grommet
Standing water in the cowl tray bridging the connector terminals
Dried washer fluid residue in the connector shell providing a conductive path
Lost 5-volt reference to the sensor, which drops the output to the bottom of its range
Sensing element failed short inside the sensor body
Corroded terminals in a connector that lives in the vehicle's water-management area
Harness damage from wiper motor, cowl panel or cabin filter service in the same space
Blocked cowl drains, the underlying condition that causes the fault to return after cleaning

Symptoms

Check engine light, often first noticed the morning after heavy rain or a car wash
Booster vacuum reading pinned at the bottom of its scale and immovable
Auxiliary vacuum pump behaviour changing — either running constantly or not running at all
Stop-start disabled without explanation
Brake pedal effort noticeably higher on applications where the module stops running the pump
Visible water, silt or dried residue in the cowl area behind the engine
Other codes from sensors sharing the same reference voltage appearing at the same time
Wiper area holding water or draining slowly during rain

Diagnostic steps

1.Get the polarity of the sensor straight before you interpret anything. Look up whether the output rises or falls with increasing vacuum on this specific application. On most pressure-style sensors low voltage corresponds to high vacuum, which means this code does not describe a lack of assist — and getting that backwards sends the whole diagnosis in the wrong direction.
2.Compare the reported value against what the engine can physically produce. A naturally aspirated engine at idle makes vacuum in a known band, and a signal below the electrical floor represents a value stronger than any engine can generate. That impossibility is the proof the number is electrical rather than real.
3.Unplug the sensor and watch what the value does. With the connector open the reading should jump to the opposite rail. If it does, the harness and the module are healthy and the fault is the sensor or its ground. If it stays at the bottom, the signal wire is shorted to ground somewhere between the connector and the module.
4.Inspect the connector for moisture and residue before doing anything else invasive. This sensor sits under the cowl behind the engine, directly beneath where water shed off the windscreen collects, and where washer fluid overflow and leaf debris end up too. A shell with a crust of dried residue in it is a short in slow motion.
5.Check the cowl drains while you are there. If they are packed with leaf litter the tray holds standing water, and any connector in that tray will short again a few weeks after being cleaned. Clearing the drains is part of the repair, not housekeeping.
6.Trace the signal lead across the bulkhead. This circuit is one of the few that has to pass through a grommet in the firewall, and a grommet that has hardened or been displaced lets the harness rub against a sheet metal edge that is more than capable of cutting through to ground.

Repair cost

$0 – $500

The floor is zero because a blocked cowl drain cleared with a length of trimmer line and a connector dried and cleaned is a repair that costs nothing but time, and it is a real outcome on this code. A shop diagnosis to reach that conclusion is $90 to $170. Cleaning and re-terminating a water-damaged connector runs $120 to $280. Repairing a signal wire that has chafed through at the bulkhead is $150 to $350, with the cost driven by access rather than by parts. Sensor replacement, where the element really has failed short, is $150 to $400. Whatever the repair, clear the drains as part of it — a connector fixed under a tray that still fills with water will short again within a season.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.